Power Brake Pedal Seems Stiff

[sick472]

I'm new to power brake troubleshooting and not even sure if I have a problem. The truck just hit the road and the brake pedal seems stiff to me. It stops the truck fine and I can lay into the pedal an lock the wheels, but it takes a lot more leg muscle than my 85 K20. So much more that when I drove the 72 for a week and then hopped in the 85...I about threw myself through the windshield.

Where should I begin looking to make an improvement?

Note: It has a new master cylinder, prop valve, and 11" booster from GMC Paul's with stock 72 front disc brakes.

[68 P.O.S.]

Here's a good troubleshooting guide from CPP to get you started. https://www.classicperform.com/TechB...oubleshoot.htm

[sick472]

Great place to start 68, thanks!

Turns out, according to the vacuum gage...I only have 15" hg max. I will check the dash mounted gage against another in the shop tonight, but I may not have the recommended 18" of vacuum. Hmmmm?

Edit: after some reading on the net...There are plenty of people with power brakes and less than 18" hg of vacuum. I guess I have some more research to do. I think I'll start with a thorough timing set-up and check, then move to the carb..then the brake vacuum.

[RodnRudy]

I had a hard pedal problem when I first got my 68 done. I also used the CPP booster assy. The brake pedal would be way too firm for the first few miles of driving and then would be normal after that. I had 17" of vacuum at the booster. I changed the check valve on the booster with a new one and that solved the problem. The CPP check valve was sticking for a bit then would free up and work normally. That would be an easy check for you, worth a try.

[Minkota]

What size Cam are you running? Too big a Cam can effect vacuum at low RPM.

[RichardJ]

How well is the booster working?

The vacuum boosters are designed so that in case of engine failure, there should still be some brake assist for three applications of the brake pedal.
With the engine running and pressure on the brake pedal. Turn the engine off. The pedal should remain in that position, but repeated application will produce a higher, harder pedal.
Hold pressure on the pedal and start the engine. The pedal should drop. The booster is multiplying your foot pressure and pushing the pedal down.

The MC used with a booster usually has a larger bore than with manual brakes. It's possible the MC bore is way too large or the booster isn't working as well as it should.

>>.I about threw myself through the windshield.<<
Elaborate on this. Does it happen at all speeds and with all applications?
If this is a low speed problem such as in a parking lot or while parking, the problem is with the Metering Valve inside the Combination valve.

Could you provide another description of, "pedal seems stiff".

[sick472]

I've done a bit of testing. I do get upwards of 20" hg while cruising, mostly down hill with light or no accelerator pedal. I did pull the check valve out of the booster last night with the engine idling and could feel/hear no suction through it, but my time ran out and I had to quit for the evening. I'll pull the check valve completely off the hose and see what it does this evening.

I am betting on a bad out-of-the box check valve (like RnR says) as the pedal feels the same no matter what...running, not running, fast, slow, etc. It reminds me of non-power brakes. They stop the truck with no problem, but only with ample leg muscle.

[Steve Cole]

What is your pedal ratio? What is the master cylinder bore size? Have you tested the booster as Richards suggested?

[scf100]

SAME problem here on my 71 c-10......

[sick472]

Originally Posted by Minkota

What size Cam are you running? Too big a Cam can effect vacuum at low RPM.

458 lift, 218 duration at 0.05° tappet lift...

[sick472]

Originally Posted by Steve Cole

What is your pedal ratio? What is the master cylinder bore size? Have you tested the booster as Richards suggested?

The pedal ratio is stock for the 72 C10. MC bore is what came with the kit from GMC Paul's for the 72 with front disc brakes.

The tests showed no signs of change on either acount...it feels like a stock non-power brake pedal. That's why I am betting on a blocked check valve. I don't think the booster is seeing any vacuum. I'll try the tests again tonight if the check valve tests to be in good order...last night was a bit of a whirlwind with some interruptions.

[un4gvn!]

I have a big block in my truck. It had the 11 inch single diaphragm booster. I always thought it was only slightly easier than manual brakes. I replaced it with a 7 inch double diaphragm booster because I couldn't get my valve cover off. Now it feels like you are in a modern truck with the power brakes. I can't believe the different it made.

[sick472]

Problem solved...I have a carb adaptor with a rear port that I had the booster vacuum hose hooked to, apparently that port is not open with my setup. I switched the vacuum line to the back of the carb and what a difference! I even scrutinized the air/fuel mixtures and got another inch of vacuum to boot.

Thanks to all that offered assistance!

Edit: It turns out that the carb adaptor has to be drilled in order for the rear vacuum port to function...I guess I skipped that step many years ago.

[Minkota]

Glad you got it figured out.

[sick472]

Thanks! Lesson learned...

A "Vacuum" booster needs ONE important thing...VACUUM!