Brake bleeding and pedal height questions

[rechinca]

All new lines in the front, reman booster, new MC and correct (factory) PB bracket. Truck is a 1971 that was manual before and crap conversion was done by PO. Booster and MC came from NAPA. Combo valve is the previous CPP PV2 unit that was used on the conversion (that would throw you into the steering wheel with the slightest pressure). TRUCK IS PARKED NOSE HIGH ON A SLOPED DRIVEWAY.

Question 1.
I can’t get fluid to bleed through to the rear lines. (Haven’t tried the front yet). I even tried a vacuum pump and nothing. I got a small amount of old fluid and a tiny amount of cleaner fluid to come out, but really about 2-3 oz total from both sides. I bench bled the MC, but as soon as I pulled the plastic plugs, fluid started coming out of the ports. Did I just allow it to fill back up with air before I could get the lines installed? What is or could be causing the no bleed? Is it air in the MC or is the slope and empty system full of air that is killing me? Is the sloped driveway causing me issues? I’ll take any shot at it. Never worked on a dry system from the start so it unchartered territory.

Question 2.
Is the brake pedal arm supposed to rest against the rubber stop on the plate under the column? Mine doesn’t even com close with the pedal just barely higher than the gas pedal. Seems that it should not be that low. On the MC side, it is a deep pocket for the pushrod from the booster. I used one of the little gauges to set the depth of the booster pushrod to about 1/16” of the MC and there is very little free play in the pedal.

I know these are easily searchable questions, but in reality a lot of guys come on here with the same questions and then never post up what the actual solution was.

[Big Kev-O]

If you stopped getting fluid out fo the rear when you were bleeding the brakes there is a good chance you "tripped" the pressure differential valve inside combo valve. One of the functions of the combo valve is to isolate the front from the rear in case pressure is lost. If it detects a loss of pressure in one side(front or rear) it stops the fluid to that side and grounds the brake light switch on the PV2. You can test to see if the valve is tripped by checking if it is grounded. You can also test it with a test light by clipping the light to the switch and then touching the positive battery terminal. If it is indeed tripped you will have to follow the instructions to reset it. To prevent this issue you are supposed to remove the switch and install a $10 threaded tool, picture below, that holds the valve in place during the bleeding process.



Link to clearly written PV2 bleed instructions:
http://mbmbrakes.com/content/pv2k-pv4kv2_Generic.pdf

As far as pedal height, mine is slightly higher than the gas.

[rechinca]

Originally Posted by Big Kev-O

If you stopped getting fluid out fo the rear when you were bleeding the brakes there is a good chance you "tripped" the pressure differential valve inside combo valve. One of the functions of the combo valve is to isolate the front from the rear in case pressure is lost. If it detects a loss of pressure in one side(front or rear) it stops the fluid to that side and grounds the brake light switch on the PV2. You can test to see if the valve is tripped by checking if it is grounded. You can also test it with a test light by clipping the light to the switch and then touching the positive battery terminal.

Link to clearly written PV2 bleed instructions:
http://mbmbrakes.com/content/pv2k-pv4kv2_Generic.pdf

As far as pedal height, mine is slightly higher than the gas.

That was one of my concerns. Makes especially fun to try and diagnose with no engine in the truck. I'll have to run a jumper to my wife's truck to get power and test the valve. That is what is what I was considering. I may try and syringe fill the lines with the bleeders open to help get some fluid down and prime them. Going to try and bleed the MC again on the truck if need be. Brakes are such an easy yet frustrating at times. Seems they always work perfect or fight you tooth and nail.

[rechinca]

I happen to have a couple of those in the tool box. I tried to use one, but maybe that should have been my first clue, it didn't screw all the way down. When I did use it on the wife's 71 F250, they didn't seat all the way down but it was on the original combo valve and there are probably variances. I was thinking I had it ok, but now probably not so much.

I don't mind my brake pedal being down that far, again referring to the wife's F250 whose brake pedal is a good 3-4" higher than the gas pedal and with a size 14 shoe it makes for a pain to apply the brakes at times.

[rechinca]

Hooked up to a battery and put the switch back in. The test light did NOT light up and the valve looks to be centered. This leads me back to possible air in the MC so I will disconnect it and have another go at bleeding it. I really hate brake systems. Simple to work, simple to not work and a pain all the way around.

[rechinca]

Possibly solved question 1. My helper (otherwise known as the wife) is teaching from home so I finally said heck with it. Valve is centered so I cracked open the lines from the combo valve and pressurized with a little air while the bleeders were open. Just a quick shot of air at 35psi. Left the LR cracked and a tube into a jar of fluid. Low and behold an hour later it started trickling. Check the RR and was able to get som fluid coming out. Will finishing bleeding in a couple of hours when the wife is out for the day.

[RichardJ]

>>On the MC side, it is a deep pocket for the pushrod from the booster. I used one of the little gauges to set the depth of the booster pushrod to about 1/16” of the MC<<

A MC with a deep pocket is used for manual brakes. A MC used with a booster has a shallow pocket. There is no way a booster pin could be adjusted to reach into a deep pocket, manual MC, unless you use an adapter plug. Something must be wrong with your description of deep pocket. I set the pin adjustment a little closer than 1/16".

>>Combo valve is the previous CPP PV2 unit that was used on the conversion (that would throw you into the steering wheel with the slightest pressure)<<

The Metering Valve portion of the Combo valve is designed specifically to control that problem. ???

>>I can’t get fluid to bleed through to the rear lines.<<

When bleeding the FRONT brakes through the Metering Valve, the metering Valve pin must be pulled OUT, so that the low pressure used during bleeding will flow through the Metering Valve while bleeding. ???


Those two ??? ??? make me think you should check to make sure the Metering Valve end of the Combo Valve is connected to the front brakes and the Proportion Valve end goes to the rear brakes.

>> I bench bled the MC, but as soon as I pulled the plastic plugs, fluid started coming out of the ports. Did I just allow it to fill back up with air <<

I'm not a proponent of bench bleeding unless the MC is mounted Nose-high. You could hold the MC with nose down when you pull the plugs, connect the lines and then tip it back and mount to the booster. Seeing as you have already started the bleeding, this is no time to pull the lines back off and let more air in.

[LockDoc]

Originally Posted by rechinca

Possibly solved question 1. My helper (otherwise known as the wife) is teaching from home so I finally said heck with it. Valve is centered so I cracked open the lines from the combo valve and pressurized with a little air while the bleeders were open. Just a quick shot of air at 35psi. Left the LR cracked and a tube into a jar of fluid. Low and behold an hour later it started trickling. Check the RR and was able to get som fluid coming out. Will finishing bleeding in a couple of hours when the wife is out for the day.

Did you replace the flex line from the frame to the rear axle? There have been many problems with the interior lining flaking off on them and blocking the fluid flow.

LockDoc

[rechinca]

Great info guys and thanks for the responses. I will try and address some questions real quick then follow up in the morning, 3am wakeup comes early.

Deep pocket MC: I am saying deep as it certainly seemed as though it were the deep pocket when compared to the illustrations I have found. Which also included several conflicting statements here where some state the deep pocket is for power brakes and the shallow is for manual. Yet I have read several posts saying exactly what you said Richard. NAPA only showed two options and I went with the one the guy recommended. These guys actually know vehicles so I tend to trust them.

Let’s just say the previous hack job at converting to power brakes made this the touchiest brakes I’ve experienced outside of my Ford Fusion when it’s cold.

I specifically called CPP twice to verify pin position when bleeding the brakes and two different guys said the opposite. They said the pin needed to be almost flush.

I hated to do it but I did the syringe method on the MC. I have done it before with some success. I did pull the lines at the MC and put the fittings/hoses back in. Using the syringe to force fluid in and air out has produced decent success and did so again.

This is one of those things that needs to be stickies when the answers are decided upon. So much conflicting info from “hey it worked for me” vs what needs to be used parts-wise. MC pocket depth vs manual/power is huge. Richard I trust what you have offered vs what I have read and the “tech” guys at CPP.

Doc I did not replace that flex line or the hard lines at the rear. I was trying to get the system up and running until I could get it under its own power and moved to a flat surface knowing I need to pull the rear brakes apart for inspection. Being on a sloped driveway I can’t safely pull the rear apart for work. Add in someone replaced the factory rear axle with what appears to be a leaf spring converted truck 12 bolt and the ground down the axle tubes. I can’t find any stamping at this time to give me any help on ID of its origin. I can only read one casting at this time due to all the crud built up and that says GM80. I know that flex line is a concern, but I felt it needed to wait until I could replace the hard lines based on how crusty that connection looked.

I work the next three days so hopefully by the time Monday rolls around the gremlins will go away.

[LockDoc]

Originally Posted by rechinca

Great info guys and thanks for the responses. I will try and address some questions real quick then follow up in the morning, 3am wakeup comes early.

Deep pocket MC: I am saying deep as it certainly seemed as though it were the deep pocket when compared to the illustrations I have found. Which also included several conflicting statements here where some state the deep pocket is for power brakes and the shallow is for manual. Yet I have read several posts saying exactly what you said Richard. NAPA only showed two options and I went with the one the guy recommended. These guys actually know vehicles so I tend to trust them.

Let’s just say the previous hack job at converting to power brakes made this the touchiest brakes I’ve experienced outside of my Ford Fusion when it’s cold.

I specifically called CPP twice to verify pin position when bleeding the brakes and two different guys said the opposite. They said the pin needed to be almost flush.

I hated to do it but I did the syringe method on the MC. I have done it before with some success. I did pull the lines at the MC and put the fittings/hoses back in. Using the syringe to force fluid in and air out has produced decent success and did so again.

This is one of those things that needs to be stickies when the answers are decided upon. So much conflicting info from “hey it worked for me” vs what needs to be used parts-wise. MC pocket depth vs manual/power is huge. Richard I trust what you have offered vs what I have read and the “tech” guys at CPP.

Doc I did not replace that flex line or the hard lines at the rear. I was trying to get the system up and running until I could get it under its own power and moved to a flat surface knowing I need to pull the rear brakes apart for inspection. Being on a sloped driveway I can’t safely pull the rear apart for work. Add in someone replaced the factory rear axle with what appears to be a leaf spring converted truck 12 bolt and the ground down the axle tubes. I can’t find any stamping at this time to give me any help on ID of its origin. I can only read one casting at this time due to all the crud built up and that says GM80. I know that flex line is a concern, but I felt it needed to wait until I could replace the hard lines based on how crusty that connection looked.

I work the next three days so hopefully by the time Monday rolls around the gremlins will go away.

Let us know how it goes on Monday.

LockDoc

[rechinca]

Just a quick photo on the MC bore depth. It measured right at 2”. Richard do you think I should be looking at another MC or is the depth ok?

[RichardJ]

A manual MC has a deep hole.
I personally, have never seen a vacuum booster that would reach down inside a manual booster. I have machined an adapter plug like the one CPP sells.

The height of the booster pin can be checked with a tool made for that purpose.
I use a straight edge and a simple depth gauge. reduce C by the gap. 1/16" or slightly less.

[rechinca]

Originally Posted by RichardJ

A manual MC has a deep hole.
I personally, have never seen a vacuum booster that would reach down inside a manual booster. I have machined an adapter plug like the one CPP sells.

The booster I bought had an adjustable rod on it and I was able to set that out the depth I measured with my gauge, but boy did I have to adjust it out a long way to make it work. That being said, all of the shallow pushrod depth MC’s like in the photo you attached that I looked up were for C20’s and not half ton. In fact none of the parts I looked up online from NAPA through all of the ones listed on Rockauto differentiated between manual or power brakes. They all list one number with no options listed. The only way I could find the shallow one was by selecting the 3/4 ton version with the thicker booster giving you the shorter rod length protruding from the booster necessitating a shallow MC. I even found a few write ups about it this morning on this site explaining the same.

So not sure I am making any headway there or the water is becoming more muddied.

Just to add insult to injury, on the correct MC, does the front port feed the front brakes therefore would the front port be considered the Primary Port?

[rechinca]

Great thing about work right now is it affords a LOT of down time. Good for me, bad for you all trying to follow along with this fun little rabbit hole.

Back to the trusty Truck Parts book (why did I EVER buy this thing). Here is an interesting little factoid: There are in fact TWO part numbers listed for 71-72 10 Series trucks. One for except brake booster and one for "w/brake booster".

I could somewhat cross-reference the one for "w/ brake booster". Part number is listed as 6273936. Running it through it the web leaves us with Dorman M80568. The pics for the part show a deep pushrod pocket in the piston and a large pot to the front and small pot to the rear.

It also crossed to a Raybestos MC36461 master cylinder. Almost identical in looks to the Dorman. Interestingly, neither of those parts come up when searching for a 1971 C10 with 350. It's maddening I tell you.

If you see something I am missing please chime in.

I think it's time to crack the bourbon as I am completely lost now.

[geezer#99]

With the m/c the po used, do you still have it?
I’m curious what the bore measurement is?

[gmc684x4]

Parts books have lost there accuracy the last decade or so condensing part numbers transferring from one data base format to anouther people not carring for there accuracy

I even found issues with newer cars nissan parts can be hard to get right from many parts places as the computers list wrong parts

My 68 has deep pocket manual master cylinder all my other trucks that have a booster have a shallow pocket most have original gm master on them

[rechinca]

Originally Posted by geezer#99

With the m/c the po used, do you still have it?
I’m curious what the bore measurement is?

Geezer that is an excellent question. I still have it. I have learned to keep parts off this truck the hard way and not turn them in for cores. I am out of town for work so I will measure up the old MC and post it up on Monday.

[rechinca]

GMC you nailed that one. The need/ability and care to carry correct parts anymore for older vehicles just isn’t there. Even my glass guy said trying to get some door glass for mid-90’s to early 2000 Honda’s and Nissans is hard to find.

I think that is why it is so important for these forums to exist. Sometimes it is a matter of just documenting what you find to try and save someone else the headache in 2 weeks or 20 years. I think it’s also why you find guys go straight to complete aftermarket set-ups just to avoid the headache. You get so many variances in the aftermarket “new” parts and so many times the manufacture made changes in the builds it’s hard to keep up with what is correct vs superseded 10x over or redesigned and have it all go back together properly.

[RichardJ]

>>Just to add insult to injury, on the correct MC, does the front port feed the front brakes therefore would the front port be considered the Primary Port?<<

On a dual master cylinder, the rear piston and port is the Primary Port and primary brake system. Force applied from the brake rod is applied to the primary piston first, but that force is applied to the secondary piston equally.

Pressure at the Primary Port and the Secondary Port is always equal.

There is no standard answer to whether the Secondary Port is used for front or rear brakes. GM switched some of their brake systems front to back, year to year during the time they were switching from drum/drum to drum/disc and even disc/disc.

FWD brake systems, especially import, use diagonal brake systems where the secondary port is connected to the left front and the right rear brake. The primary port is then connected to the right front and left rear. Because they are a dual disc/drum system, they use dual, tandem proportioning valves. Even more fun with ABS

[rechinca]

Little more gleaning of info from older threads. Finally able locate a couple places that list two different MC for the 71-72 C10's and they are separated by manual brakes and power brakes. One is LMC the other is Truck and Car Shop. Seeing how it's Saturday I did not have a chance to call and get some info on the specs. I will try and see if they want to be helpful parts folks on Monday to give some actual specs on the bore, port orientation/size and deep or shallow pocket.

Since my booster is a rebuilt unit, I can't say for certain what it should have had, but it has the long pushrod for a deep pocket in the piston. Since NAPA doesn't differentiate on power vs manual brakes, who knows what potluck rebuilt booster I have and if the rod is even correct (although from the patina on it, it at least is an older pushrod. Couldn't find my paperwork on what they sold me for sure. But she looks right so why doubt it? Kidding. As Paul Harvey said, Stand by for news!

[rechinca]

Originally Posted by geezer#99

With the m/c the po used, do you still have it?
I’m curious what the bore measurement is?

Geezer here is the old MC. Based on the stamped letters on the lid it’s an AC Deco unit. It has a deep pocket for the pushrod, 1 1/8” bore, 2 equal size chambers with the front being 9/16” and the rear being 1/2”. I can’t for the life of me find the pics of the booster I removed from the truck. But it was a Bendix style one and I matched it to the one I got in exchange at NAPA. Now because of the differences in the Delco MC and CPP lines to the CPP combo valve, it had adapters in it. Still have to call the online places and see what info the can provide.

[rechinca]

Does this explain my problem for the low pedal? From other pics I’ve seen, the upper pushrod in the back of the boosters was longer and positioned the swing arm closer to the firewall. The first two pics are mine and the next are ones I’ve screenshot from other posts.

[geezer#99]

Seems your bore size is the same.
I thought maybe it was slightly smaller causing the touchy brakes the po had.
I’ve run into bore size differences before that changed braking characteristics.

[rechinca]

Anyone can snap me a pic of their booster to bracket and the rod distance to the firewall?

[jocko]

Originally Posted by rechinca

That being said, all of the shallow pushrod depth MC’s like in the photo you attached that I looked up were for C20’s and not half ton. In fact none of the parts I looked up online from NAPA through all of the ones listed on Rockauto differentiated between manual or power brakes. They all list one number with no options listed. The only way I could find the shallow one was by selecting the 3/4 ton version with the thicker booster giving you the shorter rod length protruding from the booster necessitating a shallow MC. I even found a few write ups about it this morning on this site explaining the same.

Just to add insult to injury, on the correct MC, does the front port feed the front brakes therefore would the front port be considered the Primary Port?

The reason all the 71-72 C20 MCs show up as the shallow pocket type is because power brakes were standard on 71-72 C20s and above. You should not run a C20 MC on a C10 for the purpose of getting the shallow pocket - the C20 MC is plumbed differently than a C10's - on a C20 MC, rear MC port goes to the FRONT connection of distrib block and then off to the front brakes (with 3/16" line) and the front MC port goes to the REAR of the distrib block and then off to the rear brakes (with 1/4" lines).

If you have a C10 with power brakes, you really need to find a correct, shallow pocket 1/2 ton MC. Some folks have made or found custom length intermediate pushrods to go between the booster and MC with a non-power deep-pocket MC, and that is a workable solution, but not as workable or as simple as having the correct MC in the first place. Sometimes it's worth going to the auto parts store and asking them to drag out every 71-72 MC they have on hand and physically look at them to see if they have what you need. I completely agree with you that it is maddening that this is becoming more and more difficult to do without a bunch of searching and hoping when you order something that it will be the correct part. That's why I only drive 3/4 tons!