SharkAttack

My 70 big block 5-speed manual is a modified car with a mild cam, vacuum pump, and power brakes (among other things, but these are relevant to the question). It had great brakes when I purchased it 4 years ago, and it's not driven much.

Lately and suddenly, without me having done anything to the brakes or the car otherwise, the brake pedal still feels firm like before, but stopping power is very weak. I suspect the power brake booster. It seems to run and drive normally, so a vacuum leak is currently lower on my list of probabilities. I haven't had time yet to drive it and watch/listen carefully for idle changes when in neutral and braking.

What typically causes brake booster failure?
More importantly, what's the procedure to verify the issue is, or is not, the brake booster?

'75

Put your foot firmly on the brake pedal and start the car, if the booster is functioning, the pedal will go down another inch or so.

kansas123

I read this in a recent post, however no one seemed to refudiate it (think I have read it before also):

"If no leaks I'd say it's a master problem most likely. A defective booster would just give you a hard pedal."

Post:
https://www.corvetteforum.com/forums/c3-tech-performance/4211097-1978-no-brake-pressure-again.html

I have also read something about unhooking the vacuum hose from the booster to the back of the carb/intake manifold, plugging it, and seeing what that did to the brake pedal (run the car but don't drive it). I could not find the post I read that in though. Did you do any work on the brakes or did this just start happening?

SharkAttack

No changes made to the brakes or the car, this just suddenly started happening. I updated the original post with this info as well.

stumpshot

I'd say you are experiencing manual brakes after being used to power. The booster is vacuum operated so unplug it and see if anything changes with the brakes. My money is on a failed booster.
Good luck!

3X2

A couple of things to check: 1-Is there sufficient vacuum at the booster? 2- With the engine running, slightly depress the brake pedal and listen for a hiss noise indicating a torn diaphragm in the booster. I have seen this many times on C5-6's with larger cams. Pedal is hard and stops poorly if the brake pedal is pushed multiple times in a row, especially at idle, before the booster can recover. Driving, there isn't any concerns.

DUB

I agree it sounds like it is the booster from what has been described in how it reacts.

I have also seen a faulty check valve in the booster that caused it to not work well and when i replaced it....the brakes worked correctly.

DUB

cagotzmann

Here is how I tested by brake booster on or off the car. It should hold vacuum, mine held 15Hg for a good 30 minutes.
It does take many pumps to get to 15 HG with this little hand unit.

jackson

Rubber brake hoses from chassis to caliper can and do age-out, then swell internally and collapse internally; not noticeable (usually occult) from exterior. Can and do present described symptom. Many folk don't consider replacing until exterior looks rough. Good name brand (eg raybestos, Wagner etc) Rubber replacements from local parts house will likely outlast driver... Teflon & Stainless braid not important. Not saying hoses are culprit...but may be.

stingr69

Had that one time and it was fluid getting past the seals and on to the pads. Changed calipers and still the brakes did not grab like they should. Replaced the pads and all was well.

buns

Some good info here:

http://mbmbrakes.com/brake-booster-diagnostics/

sambrand

+1 agree, sounds as if booster may or may not be the issue, prop valves can pick up random junk from the lines and be partially plugged or totally plugged, and not always consistently so if the blockage piece of junk is loose inside the valve this can cause a fluid volume check at the calipers to pass yet a day or hour later the fluid may become constricted again.

mrvette

I not saying why or HOW, but I had a 'failure' in the front seal of my master cyl, and it was the prime pressure for the front brakes, long since previously I had removed that 'proportioning' valve from the system....

I have had Hydro Boost brakes since maybe 15? years now, and brakes solid as a rock, so when the seal 'failed' the front brakes were not applying.....brakes in the rear were stopping the car, and pedal travel was huge.....but the car did stop, I still have the 'failed pistons and seals'......

SharkAttack

Finally had time to investigate further. I have a vac pump, my check valve is built into the elbow that inserts into the brake booster, and it works properly (only allows suction out of booster). Brake booster hose goes to a small 2-port manifold, and the hose on the other side of this manifold goes to the vac pump. This manifold has an electrical wire to it, so I'm guessing its electronically switched to when the ignition is on. Never had or seen a vac pump on a C3, so this part is all new to me. Vac pump always comes on each time I start the car, then shuts off after a period of time (as it should).

My testing procedure: Started the engine and let it warm up. Shut engine off. Started engine with foot on the brake pedal, and the pedal depressed more than the 1" or so it should have. In fact, it almost went to the floor. Did this a few times, and one time, even the red "brake" light came on when the pedal was fully depressed. Have a very old vac gauge that was my dads, not sure how well it works, but when connected to the booster line it reads 21 and every 20-30 seconds will bump up to 22 then slowly move back to 21. After shutting off the engine, the vac gauge shows the vacuum slowly bleeding off and ultimately moved back to zero. Still troubleshooting so more to come, because it now appears I have air in the line and/or a leak.

HeadsU.P.

Most people are confused about what exactly a booster does. Its an assist. It makes it so you can apply the brakes barefoot if one so desires.
A brake pedal to the floor has nothing to do with the booster. A brake pedal that is high off the floor then drops a little upon starting the vehicle is normal. But a pedal to the floor is a leak somewhere regardless if there is a booster in place or not. (PB or non PB)
That's why, after performing any brake work, leave the engine off, stand on the pedal. It should not budge.
You have a leak somewhere. Worry about that first, then worry about the Vac / booster system. The red warning light means you lost pressure. That red light doesn't know or care if you have power brakes.

SharkAttack

Removed all of the wheels, found the cause. Small, slow leak in one of four pistons of the LF caliper. When I checked a few weeks ago, there was never any brake fluid on the wheel, tire or ground. Glad it's not the booster (yet).

HeadsU.P.

Ok, that's not too bad. Now what? Rebuild, buy rebuilt, buy new?
When you install the replacement caliper, you really should bleed both fronts. But the rears will be fine, being on a separate circuit.

SharkAttack

Thx HeadsUP. No confusion on my part, a booster simply multiplies the force you apply to the brake pedal. And yes, when my pedal suddenly went to the floor yesterday, I knew I had a leak. Fortunately that's easier to fix than replacing a booster!

Monterey C6

I know this by experience. Also I test drove a 75 Vette last summer at a dealer and that car had the hard peddle and poor break performance. Turned out to be a froze proportioning valve.