Pedal too firm after brake overhaul

[chevy72blu]

My truck sat for quite some time and was giving symptoms of a bad M/C. On the good advice from a few board members, I decided to give the system an overhaul.

I replaced the booster and master cylinder as well as the 3 rubber hoses and thoroughly bled the system (old fluid was nasty stuff). The pads and shoes were in good shape (recently replaced prior to sitting) and the wheel cylinders were not leaking or have visible issues, so I did not change them.

The truck now seems to require much more pedal pressure to stop the truck than before ie: the pedal is too firm. The booster seems to be functioning correctly (pedal falls 1/2-1" after being pumped up and then having the engine started). There are no leaks or anything visible that I can find. The truck will still lock up the front wheels just fine but it requires a great deal of pedal pressure.

The truck has front disk/rear drum and all the components I replaced are correct (which took some time to figure out on my mixed bag of parts). Reman Delco 3/4 ton 10.5" dual booster, new brake best premium (OReilly store brand) M/C, and brake best flex lines. Synthetic DOT 3 fluid.

Can a booster be bad even if the pump-up/engine on test passes?

[TBONE1964]

Give it some time. Drive it and put the brakes through some heat cycles and see if anything changes.

A good test for the booster is to apply the brake while driving and on the gas. Lets say you are going 50 - 60 mph on the gas and apply some brake. The pedal should be very hard to push until you let off the gas and then it should feel normal.

It sounds like you covered the basics fairly well.

[airdale94]

I just replaced a original booster on my 3/4 ton cause the pedal was hard with reman Cardone from O'Reilly's and it's a "little" better. Sometimes it works perfect. Different weather conditions? sitting for awhile? The old one was hard all the time. I just haven't pulled it off and warrantied it yet.

[chevy72blu]

Copied from CPP:

Test Power Brake Booster

If the pedal feels "hard" while the engine is running, the booster isn't operating correctly. If you suspect the booster is defective, do not attempt to disassemble or repair the power booster. Doing so is unsafe and will void your warranty.

Test 1

With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the booster is working properly.
Test 2

Run the engine a couple of minutes.
Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster.
Test 3

Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight.
Inspect the Check Valve

Disconnect the vacuum hose where it connects to the intake manifold. Do not disconnect the vacuum line from the booster. Air should not flow when pressure is applied, but should flow when suction is applied. If air flows in both directions or there is no air flow, the valve needs to be replaced.
Verify Enough Vacuum

Check the operating vacuum pressure when the engine is at normal operating temperature. There should be a minimum of 18 in. of vacuum. Vacuum may be increased by properly tuning the engine, checking for vacuum leaks and blockages in vacuum lines.

Test #1, 2, and 3 all check out. The check valve is new and is working correctly as well. The motor is a stock LS1 and idles at around 19 in of vacuum according to my scan tool (of course, that may not be reaching the booster. Should I try a vacuum gauge there?)

I will try driving it (slowly) and see if it improves. My other thought is possibly a larger bore M/C piston? I didnt compare the two when I swapped them...

Thank you for the responses!

edit: Rockauto only lists a 1.25" bore on the M/C for a C20. 1.25" would be for a C10. C30 also lists 1.25", so I dont see how there could be a mixup.

Double edit: When I bench bled the MC it was VERY stiff. Perhaps that has something to do with it and it will loosen up with driving?

[lazypineapple]

try with truck running pull vacuum line from booster. should run rough and maybe die, and you will here a sucking sound from intake vacumm,
if so,
then you got good vacuum.
you could drive around the block with booster disconnected, then connected, and see the difference,
if booster is a problem.

[cdowns]

did you set the clearances with the pushrods for both the master cylinder and power booster

[chevy72blu]

lazypineapple,

Removing the booster line nearly killed the motor. I touched my finger to the hose and thought I was going to be sucked in! Its definitely pulling plenty of vacuum.


CDowns,

Can you give me some more info? I have seen that referenced in other threads but I'm not sure what that exactly means. The pushrod for the booster was the same length as the original - it bolted right up. I compared the M/C rod between the old and new and they were close to the same size (new rod is perhaps 1/16 shorter). I just bolted them up with no adjustment.

[SS Tim]

IIRC there was a previous question about the booster bracket that I lost track of.

Now you had a single diaphragm booster, correct?

Is the booster bracket assembly interchangable between a single and a dual diaphragm booster?

Finally my point, is the relay linkage inside the bracket the same ratio between the two?

[chevy72blu]

Tim,

You are correct about my previous questions. On your advice from that thread, I did some more digging and found the correct (stock) equipment.

The booster is a 10.5" unit that I believe is double diaphragm (stock on 3/4 tons and 1 tons). The 1/2 ton unit is a few inches thinner and has a different bolt pattern on the bracket.

The correct one for my truck (which I installed) is this type:



The 1/2 ton booster (which does not fit) is this style:



Pics borrowed from here: http://outintheshop.com/faq/Brakes/brakeinfo.html
More info on the differences is on that site

I can take pics of my setup as well. I have a new stock style 1/2 ton booster that a board vendor mistaken told me was correct. They insisted that GM never made a different type of booster for C10s and 20s/30s

[chevy72blu]

Update: Went out for another drive (unpopulated area) and it seems to be somewhat better. I had my wife drive so I could watch the pedal and it looks to be traveling a good 2" with light input before slowing the vehicle. Past 2", its very firm.

Perhaps the issue could be adjustment like cdowns mentioned? If someone has instructions on how to do this I would really appreciate it!

I bled until I saw no air (full quart) but perhaps there is still some trapped?

[SS Tim]

OK thanks for clearing that up. So even if there is a ratio difference in the relay you have the correct application. Running out of ideas here.

[chevy72blu]

Originally Posted by SS Tim

OK thanks for clearing that up. So even if there is a ratio difference in the relay you have the correct application. Running out of ideas here.

I cant imagine that the throwout ratio is different with the correct part.

I'm running out of ideas also... lol. I'm really leaning towards it being a stubborn bleed. I bled for a LONG time, way past the brake fluid running clear and no more air bubbles in the clear bleeder line on each wheel.

As I understand it, any manual bleeding method is bested by power bleeding. Maybe building a power bleeder or having a shop do it again is the way to go

[slikside]

Originally Posted by chevy72blu

I cant imagine that the throwout ratio is different with the correct part.

I'm running out of ideas also... lol. I'm really leaning towards it being a stubborn bleed. I bled for a LONG time, way past the brake fluid running clear and no more air bubbles in the clear bleeder line on each wheel.

As I understand it, any manual bleeding method is bested by power bleeding. Maybe building a power bleeder or having a shop do it again is the way to go

A firm pedal is not usually associated with air in the line/insufficient bleed. To the contrary, a spongy, or soft pedal is.

I can't say for sure what your issue is exactly, but from your description, it doesn't seem to me to be one of bleeding. It sounds like you bled it well. When I did mine I only gravity bled it, and that worked just fine. Lots of other folk on here have done it the same way. And if, by chance you still had a bubble in there somewhere, you wouldn't have a firm pedal.

[chevy72blu]

Originally Posted by slikside

A firm pedal is not usually associated with air in the line/insufficient bleed. To the contrary, a spongy, or soft pedal is.

I can't say for sure what your issue is exactly, but from your description, it doesn't seem to me to be one of bleeding. It sounds like you bled it well. When I did mine I only gravity bled it, and that worked just fine. Lots of other folk on here have done it the same way. And if, by chance you still had a bubble in there somewhere, you wouldn't have a firm pedal.

Yes, that is my understanding as well. What I am confused about is the fact that the pedal has to travel a good 2" before beginning to slow the vehicle - THEN it gets very firm.

Still scratching my head... I'm not above taking it to a brake shop but going to a new place + having no clue whats wrong is a good way to get jerked around.

[chevy72blu]

I swapped out the booster rods which I mentioned earlier.



The rod on the right is the old one, the left unit is "new" but is heavily pitted and approx 1/16" shorter than the old unit (bad angle to see the size difference).

The "old" booster rod seems to be somewhat better - pedal feel is the same but the brakes engage slightly quicker.

I'm going to try plugging the booster line and see if it feels much different on the road without a booster. I've never driven a car with manual brakes, but I imagine this is what they feel like.

Edit: The booster is definitely doing something. With no booster it wouldnt be safe to drive around the block. Now I'm back to my original question: Can a booster only be working partially as well as it should?

[luvbowties]

Originally Posted by chevy72blu

I swapped out the booster rods which I mentioned earlier.



The rod on the right is the old one, the left unit is "new" but is heavily pitted and approx 1/16" shorter than the old unit (bad angle to see the size difference).

The "old" booster rod seems to be somewhat better - pedal feel is the same but the brakes engage slightly quicker.

I'm going to try plugging the booster line and see if it feels much different on the road without a booster. I've never driven a car with manual brakes, but I imagine this is what they feel like.

Edit: The booster is definitely doing something. With no booster it wouldnt be safe to drive around the block. Now I'm back to my original question: Can a booster only be working partially as well as it should?

Our shop recently had a similar problem on a '70 vette--customer complained it took both feet to make it stop quickly! New booster and mc did no good. After 2 days experimenting/troubleshooting, we finally changed the front pads(which looked just fine) and it stopped perfectly. Probably the biggest cost to our shop for a set of pads ever--but no way to charge customer for our mistakes! Not a great cost to try new pads on your truck. Luck to you.
Sam

[chevy72blu]

Thanks luvbowties. Its certainly worth a shot at this point.

My confusion is that before the booster/mc swap it actually stopped considerably better (with less pedal effort)

[TBONE1964]

Originally Posted by luvbowties

Our shop recently had a similar problem on a '70 vette--customer complained it took both feet to make it stop quickly! New booster and mc did no good. After 2 days experimenting/troubleshooting, we finally changed the front pads(which looked just fine) and it stopped perfectly. Probably the biggest cost to our shop for a set of pads ever--but no way to charge customer for our mistakes! Not a great cost to try new pads on your truck. Luck to you.
Sam

I can also tell you that after over 30 yrs as an auto tech, there has been several occasions where a "new" set of pads had a problem like this right out of the box. I would not say with 100% certainty that it is your problem but it could be. It is not common but it is possible.

[Mr Chevorlet]

I had this problem on a 1988 chevy truck 5 years ago and never could figure it out. It took both feet on the pedal to stop the dang thing, got rid of the death trap. I would really like to know the remidy when you find out what it is.

[chevy72blu]

Well I am glad to hear that I'm not the only one with this issue. My mechanic mentioned that it could be an issue with the prop valve - perhaps too much is being diverted to the rear only.

It has to be something with the booster/mc as the problem started when I swapped them (although, the brakes were quite weak to begin with which prompted the overhaul).

Swapping the booster isnt too time consuming, and being that its lifetime warranty I suppose it makes sense to change. I will update this post with what I find. I'll try pads and shoes, next.

If THAT doesnt work, it may just be a good time to swap to the hydroboost unit I pulled out of a scrap yard a few years ago.

[luvbowties]

Originally Posted by chevy72blu

Well I am glad to hear that I'm not the only one with this issue. My mechanic mentioned that it could be an issue with the prop valve - perhaps too much is being diverted to the rear only.

It has to be something with the booster/mc as the problem started when I swapped them (although, the brakes were quite weak to begin with which prompted the overhaul).

Swapping the booster isnt too time consuming, and being that its lifetime warranty I suppose it makes sense to change. I will update this post with what I find. I'll try pads and shoes, next.

If THAT doesnt work, it may just be a good time to swap to the hydroboost unit I pulled out of a scrap yard a few years ago.

Don't forget, you said the m/c did not feel right when you bench-bled it.
Sam

[mcmlxix]

Did you ever figure it out?

[E.TexasTwister]

I'm having the same problem with a firm pedal. Did you get yours figured out?

[Steve Hafner]

. . . .

Here's something from another thread that I'm going to try on my 70:

. . . .

[LEEVON]

Boosters are bad out of the box more often than you might think...also the brake best select pads are iffy. Those points aside, I would bleed the master cylinder again. Since it's on the truck level it and return clear hoses into each reservoir, one at a time while blocking off the other port. You may still have a small bubble in one. Sound like maybe your front brakes have a bubble and the rears are the only active circuit.