'68 GMC 3/4 Ton Disc Conversion

[Droff]

I'm close to what I thought was getting my truck swapped over to front disc. I bought some '86 3/4 ton brake parts and mostly used just the spindles and control arms, everything else is new - rotors, calipers, booster and MC as well as ball joints, bushings, springs and steering parts.
I decided to replace the rear brake line because the flare nut wouldn't thread into my new prop valve. It's the line that runs from the prop valve to a union along the frame. It's a 1/4" line and one end looks to be 9/16"-18 and the other is 7/16"-24. In researching what those sizes were I ran across more MC discussion.
I have a 3/4 ton and the discussion was prop valve and MC connections and sizes. Basically, I'm now not sure I have what I should have concerning the prop valve and MC.
I picked these up a while back..

Booster and MC;
https://www.summitracing.com/parts/sum-760207

Prop valve;
https://www.summitracing.com/parts/clp-pvks-2

I don't plan on any towing, at least nothing on a regular basis and there won't be much hauling.
Am I installing the correct or incorrect parts?

Thanks.

[stsalvage]

I'm using my 3/4 ton A arms and using the CCP spindles with the stock A arms.Might have to swap out tie rods.And use moog upper and lower ball joints and control arm bushings

[Andy4639]

I would just buy the adapter for it and use it.

That prop valve I wouldn't recommend it to use on a toy push car. I swapped my 71 over to all disc with it and it blew that front rubber out and I ran a stop sign with no brakes what so ever. I ended up getting a nice GM one with no plunger in front.

[Droff]

I've got all new rubber lines as well as keeping drums in the rear, maybe that'll help with your possible prop valve issue in my application??

I wasn't sure on the prop valve, mainly due to the way the 3/4 ton is connected differently than the 1/2 ton, as the prop valve connections look to be swapped. I've already pulled the brake line, just waiting on delivery of the new line to get it installed.

When you refer to the plunger in the front, is that for the booster?

[Droff]

Trying to finish up my drum to disc conversion, for now staying with the prop valve I linked above. I've got everything connected, all new components, mostly just OEM replacement parts.
Once I got some pressure on the lines, I started getting seepage from the banjo bolt on the right caliper on the caliper side of the banjo bolt. I've got washers on both sides of the rubber line connector.
I've swapped out the crush washers 3-4 times already, cleaned the hose connector surface, pulled the caliper and cleaned that surface also. I've installed two washers on the caliper side to give that a try. There are visible rings on the caliper where the washer meets the surface and they are getting pressed into the washer, although not as deep as on the bolt head side.
The seepage is enough to get the surface shiny but not enough to get much of a drip - yet.
Any more suggestions other than a new caliper?

[Droff]

Well, it looks like I got the seep resolved. After screwing around way too much yesterday on this, I went ahead and tried the annealing of the washer suggestion. Torched it up to nice and orange and then quenched it in water.
Snugged the banjo bolt nice and tight and no leak. Worst part was losing brake fluid down the inside of my tire. Maybe a 5 minute fix in total.

On to the bleeding. I was basically just doing a gravity bleed, every now and then pump the pedal a few times and make sure the MC remains full. I've been getting a lot of clear fluid out but then I get a nice long line of small air bubbles. It kept going like this, back and forth, until I ran out of brake fluid to add so I had to stop. Is it typical to get air bubbles like this for up to 10-15 minutes doing gravity bleeding? I have no leaks I can see anywhere now but it just looks like the line is sucking in air from somewhere.

[demian5]

Originally Posted by Droff

Is it typical to get air bubbles like this for up to 10-15 minutes doing gravity bleeding? I have no leaks I can see anywhere now but it just looks like the line is sucking in air from somewhere.

Yes, I recently did all 4 corners on a 69 chevelle and I couldn't get a good pedal until I gravity bled it. Pretty frustrating.

[Droff]

I've had several 'seeps' at different connections that looks like I have now resolved. I'm still getting a nice line of air bubbles running through the tubing when I bleed all the corners and I'm not so sure it isn't the bleeders themselves letting air through when they're loose. I have a solid pedal but not sure on the prop valve, if it's actually centered. The dash brake light is a bit schizo right now, it's off/on. I was planning on a short road trip but had some other liquid escape captivity.

I replaced the steering gear box, so it's a new reman. It's been installed, new rag joint and fresh fluid. I filled the pump and after starting the truck, turned the wheel lock to lock to burp it and after about 4 times, the gear box puked all the fluid out of the bottom (opposite of the rag joint).

Any help - other than R&R?

[Droff]

It's been a bit cold and wet the last week or so but finally got the steering box swapped out with a replacement, got it connected, burped and no fluid in the drive. Small miracle.
That issue seems to be resolved but still having issues with the disk setup.

I'm leaning toward the prop valve being the problem, but not for sure. I can get the rear drums to bleed okay, the pedal drops farther once I crack the bleeder (I'm utilizing a pedal pusher). The fronts are a different story, very little fluid runs out and there is no pedal movement when the bleeder is cracked.

My thought is the prop valve not being centered. I've tried a few things to remedy that but not much luck. I tried opening both rear bleeders and pumping the brakes several times as well as mashing on the pedal with all the bleeders closed. Neither seems to have helped.

Back to the prop valve, I have a 3/4 ton and everything I've seen has the rear of the valve connected to the front MC reservoir and the front of the valve connected to the rear of the MC reservoir. Should the prop valve linked above work as plumbed or would I need to move the lines so they are front to front and rear to rear?

Thanks.

[Steeveedee]

Make sure that you have the front and rear brakes supplied from the correct sides of the master cylinder.

[Droff]

Originally Posted by Steeveedee

Make sure that you have the front and rear brakes supplied from the correct sides of the master cylinder.

Right now, the front of the MC supplies the rears and the rear of the MC supplies the fronts. I believe this is correct but if not, I'd appreciate some input on it. I'm going to re-plumb the PV to the MC and see if that helps resolve anything.

Once I get that done, it will now be the front PV connection to the front of the MC and the rear PV connection to the back of the MC. Need to pick up some more tubing today.

Thanks.

[Droff]

After a few tries.....I got some tubing bent and then got the PV connected to the MC. Lots of fluid all over but it looks like everything has held as far as leaks are concerned. Went through some gravity bleeding as well as having a pedal pusher. Pedal was pretty firm and held up overnight. Went for a drive and the pedal went to the floor after about the 3rd stop, I felt kind of a pop and then... not much.

Got home and checked the PV based on the procedure in the pic attached. I was reading voltage (about 10VDC) so it looked like the PV wasn't centered. After the procedure, voltage went away. Bled some more and the pedal was nice and firm, almost hard.

I then started the truck and the pedal went a good bit softer. What's up with that? A vacuum issue?
I went for a drive regardless and the brakes worked fine, help up with a couple of hard stops, my concern is the pedal travel before it actually bites.

So why would the pedal feel softer after starting the truck?
How far should my pedal travel before I get a good feel that the brakes are engaging?

Thanks.

[Mike C]

Once the truck is running you get some assist from the booster. I think what you are describing is normal?

[Aus69c20]

My disc brake converted 3/4 ton has a good bit of pedal travel before the brakes really grab.
The first half of travel it brakes only lightly. It annoys me also but it may we’ll be just how they are.

[Droff]

Originally Posted by Mike C

Once the truck is running you get some assist from the booster. I think what you are describing is normal?

Trying to be optimistic, sure it is.
I don't really know but that's what I was thinking.

Originally Posted by Aus69c20

My disc brake converted 3/4 ton has a good bit of pedal travel before the brakes really grab.
The first half of travel it brakes only lightly. It annoys me also but it may we’ll be just how they are.

That sounds pretty much right on with what I've got going. Not much brake at all for at least 2-3" and then it grabs in less than that.
I guess I'll see how it goes. I dropped it off at the brake and alignment shop this afternoon, I'll ask him to check things over when he aligns it.
Thanks.

[dozerbill72]

Have you adjusted the rear brakes? If they are out of adjustment it will cause increased travel in the pedal.

[Droff]

I've worked on the rears but did not adjust them, replaced the wheel cylinders and cleaned them up a while back. I hadn't thought of that as a possible cause.
Next on the list this week.
Thanks.

[RichardJ]

>>Right now, the front of the MC supplies the rears and the rear of the MC supplies the fronts.<<




If that statement applies to that picture, you have got a real mess.

That Combination Valve is comprised of three separate and independent components.
The rubber cap covers the Metering Valve Pin and must be in the line to the front disc calipers. The Proportion Valve is on the other end and must be plumbed to the rear brakes.

[Droff]

Originally Posted by RichardJ

>>Right now, the front of the MC supplies the rears and the rear of the MC supplies the fronts.<<

If that statement applies to that picture, you have got a real mess.

That Combination Valve is comprised of three separate and independent components.
The rubber cap covers the Metering Valve Pin and must be in the line to the front disc calipers. The Proportion Valve is on the other end and must be plumbed to the rear brakes.

Yes, that is initially how I had it connected. You've got my posts a bit out of order, which wouldn't be too hard to do.
I swapped the connections (the pic in your reply) so they are as you're stating. The pic is how I have it plumbed now. Front of the PV and MC go to the calipers in the front. The rear of the PV and MC go to the drums in the back.

[RichardJ]

OK, for that.

>>Not much brake at all for at least 2-3" and then it grabs in less than that.<<

There was some discussion between you and some others, somehow partially involving the booster. I couldn't really connect the dots so let me offer this.
The boosters are designed to provide three strokes of the pedal after an engine dies so you still have some brake assist to get to the side of the road.
With the engine off, push on the pedal three or more times. The pedal will get higher and quite firm. While still pushing on the pedal, start the engine. As the engine builds vacuum, the pedal will drop about an inch or sometimes more, but that just means it is assisting your foot pressure.

Some of the bleeding problems you had can be attributed to the Metering Valve. The Metering Valve restricts fluid flow to the calipers until pressure reaches some preset threshold pressure. Often somewhere around 60 psi or higher.
On your Combination Valve, when the Metering Valve opens, you will see the pin extend outward. Pop the rubber cover and have someone push the brake pedal. The pin should extend, then retract when they release the brake.
Some of the early Metering Valves worked the opposite way. They retracted under pressure.

There are tools for holding the pin out, to make bleeding easier.

[RichardJ]

In your first post,
>> I bought some '86 3/4 ton brake parts and <<
then
>>I decided to replace the rear brake line .......................the line that runs from the prop valve to a union along the frame.<<

Didn't that place the Combination Valve on the cross member under the radiator? The LMC image shows the Combo valve under the radiator, but the brakes aren't positioned correctly. The lines go through the frame in front of the brakes.
I guess you didn't get that line.
After your initial post, I had high hopes that you would.

There are many thousands of posts on this forum, regarding guys having trouble finding the right flex hose so that it doesn't drag on the tire, get pinched or pull the end off under full suspension extension.
Everyone tries to use the drum lines and routing and then always, always have trouble getting the various parts to connect and work well together.

In my opinion, let Chevy do the engineering. They're much better at it.

In my case, I used '76 parts and it bolted together like I had a '76. The holes for the lines through the frame, connecting to the flex lines was already there.

The image below is just one example in thousands showing the problem of using the drum brake line that runs across the suspension cross member.

[Droff]

Originally Posted by RichardJ

There was some discussion between you and some others, somehow partially involving the booster. I couldn't really connect the dots so let me offer this.
The boosters are designed to provide three strokes of the pedal after an engine dies so you still have some brake assist to get to the side of the road.
With the engine off, push on the pedal three or more times. The pedal will get higher and quite firm. While still pushing on the pedal, start the engine. As the engine builds vacuum, the pedal will drop about an inch or sometimes more, but that just means it is assisting your foot pressure.

Some of the bleeding problems you had can be attributed to the Metering Valve. The Metering Valve restricts fluid flow to the calipers until pressure reaches some preset threshold pressure. Often somewhere around 60 psi or higher.
On your Combination Valve, when the Metering Valve opens, you will see the pin extend outward. Pop the rubber cover and have someone push the brake pedal. The pin should extend, then retract when they release the brake.
Some of the early Metering Valves worked the opposite way. They retracted under pressure.

There are tools for holding the pin out, to make bleeding easier.

All good info I hadn't found yet. I can check out the valve once I get the truck back to the house. I do know the pedal does firm up when pumping the pedal - with the engine off. As far as dropping when started, haven't done that test yet.

Originally Posted by RichardJ

In your first post, "I bought some '86 3/4 ton brake parts .... and then decided to replace the rear brake line .......................the line that runs from the prop valve to a union along the frame."
Didn't that place the Combination Valve on the cross member under the radiator? The LMC image shows the Combo valve under the radiator, but the brakes aren't positioned correctly. The lines go through the frame in front of the brakes.
I guess you didn't get that line.
After your initial post, I had high hopes that you would.

My '68 had a combo valve up next to the MC, my guess is that some previous owner had done some type of work to the brakes. Since I was going to replace it, I wasn't too concerned with the current location. The '86 setup I picked up had the PV next to the MC, or at least close by, it wasn't on the frame either. I did not swap out the cross member so I'm using the original cross member, and the line I replaced originally runs along the backside of the cross member.

Originally Posted by RichardJ

There are many thousands of posts on this forum, regarding guys having trouble finding the right flex hose so that it doesn't drag on the tire, get pinched or pull the end off under full suspension extension.
Everyone tries to use the drum lines and routing and then always, always have trouble getting the various parts to connect and work well together.

The flex line is different story though, I've got the same issue you're indicating concerning how the lines get bolted to the control arm. I'm not sure how the drum brake line along the cross member causes that issue though, it's kept in its' original location.

Originally Posted by RichardJ

In my opinion, let Chevy do the engineering. They're much better at it.

I agree with that, I just wanted to go with discs in the front but may not have taken the best overall path - at least for me. If I would do this again, it would be a kit with all the parts instead of trying to make sure I have everything by doing my own research.

[Hotcorner]

Sorry for the high jack, but I was under the impression that my 69 k25 rear m/c reservoir was for rear brakes.

[Droff]

I got the truck back this afternoon. Alignment was done but he couldn't remove all the camber so I've got about a degree of positive camber. Removed all the shims and that's what he had left. It doesn't sound like a big deal but anyone have any input on this, other than make sure I rotate the tires?

He did check the brakes.. adjusted the rears, which weren't grabbing and bled the system one more time, got little to no air out. Tightened the wheel bearings too. I guess I was a bit too worried about getting the bearings too tight.

The pedal feels really good and it grabs sooner than it did before. I think the brakes are good at this point. Thanks for all the inputs.

Sounds like I may need a water pump now..... awesome.