Seeing all these brake threads got me really thinking about my own marginal brakes. Specifically 69427 seems to have a similar issue (although he has a vastly different set-up!). I have the long soft brake pedal. Nice and firm with engine off, but long and soft with engine running. The car still stops, but feels like stopping my 36 Plymouth...plan ahead or stand on the pedal (bit of an exaggeration, but they aren't great).

My setup is pretty much stock for a 78, minus the drilled and slotted rotors. I am using a booster from Advance and just got to wandering if anyone else has had success with a parts house booster? Is there a significant difference between what you get at the parts house and what the vendors sell? Could a poor quality booster be the root of my problem?

I need to plug the vacuum to the booster and go for a drive, but I am confident I will have normal feeling manual brakes, as the pedal is consistently firm with the engine off.

Any thoughts on the parts house booster?

 

My booster is a new Chinese-made replacement that actually works better than the OEM booster and only cost $87. So the parts house boosters can be as good or better than the OEM boosters and can cost a lot less as mine did. But a soft spongy brake pedal means you still have air in your brakes.

 

It is only soft with engine running. Consistently firm as manual brakes

 

What happens with the brakes with engine off is irrelevant.

SO...trying to read what you wrote...when you apply the brakes it requires excessive effort ...thus applying A LOT of pressure to try to stop the car....correct??????

Check the vacuum check valve in the booster and make sure that when you apply vacuum to it that is allows you to do so EASILY. I have had a bad check valve that was really hard for me to draw air through and when I changed it...it fixed the SAME problem that you have. REMEMBER this is a ONE-WAY valve...you should not be able to get air to go in both directions. I can not type out how I do it...due to some people would have way too much fun in how it would be typed out....but I am sure you can test it using your lung pressure...if you get what I am writing...so think of how you would use your lungs to fill balloons and use a soda straw effectively....if that helps.

ALSO...if you want to. Start the engine and check your brakes to get a feel for them (pedal effort to stop). THEN..turn off the engine...remove and PLUG the hose going to the check valve in the front of the booster ( assuming that you have GOOD engine vacuum going to it). Crank the engine ..put your foot on the brakes and put it in gear and just let it roll a little bit and see if the problem is THE SAME.....if it is the same....thus requiring a lot of pedal effort to try to stop it (like a freight train) ...and...it is NOT the check valve...then it is the booster is where I would start...and this does not matter if it is a new booster or not. I have seen had bad NEW boosters....right out of the box.

DUB

 

Thanks DUB. Yes you seem to understand the problem. I have not drive the car with booster disconnected yet, but rolling back down the driveway it stops well under manual braking. I pulled the valve and inflated a balloon then drank a soda . The balloon wouldn't inflate for me, but I had a nice soda straw. Not sure if it would affect it or not, but I will check the gaskets next, MC to booster and booster to FW.

 

Having a nice soda straw is really good. That at least confirms that the engine can draw vacuum and the check valve is not hindering it.

NOW...when the hose is removed and plugged..and IF the pedal is the same with the vacuum hose connected and not connected...then it is the booster...at least that is what I would replace next.

DUB

 

Thanks DUB. I hope to get to play with it this weekend. I do know, however, that when the engine is off the pedal feels different, so I expect the same feel with car running and vacuum disconnected. Perhaps I misled you before, the pedal is indeed softer feeling with car running, but it goes further to the floor and must use excessive force to get a solid stop. With the engine off, I have a good firm pedal with minimal travel (maybe an inch).

Stopping effort is the same running or not, but the pedal has more play and mushiness with it running. Stopping force either way is what I would expect from a NON power brakes.

I will do some more trouble shooting this weekend and report back.

 

Did you check proper pushrod length with the new booster?

https://www.corvetteforum.com/forums...djustment.html

 

Nope...I'll check that. Thanks.

 

Driving the car with the vacuum disconnected is not that same as driving with traditional manual brakes. Manual brakes use a smaller size bore in the master cyl. which generates the pressure needed to stop the vehicle.
I have driven a pb vehicle and when the booster went out,I pulled myself off the seat applying pressure to the pedal and death gripping the steering wheel. It actually made it worse than normal brakes, I don't recommend you try it. I replaced the booster and the vehicle stopped like when it was new.

 

Your brake pedal will act completely different when the engine is running and NOT running....this is due to the vacuum.

Perform that test I wrote...IT IS IMPORTANT.....because if your effort to stop IS THE SAME ...when the hose is attached..and when it is disconnected and plugged ...I need to know.

This does not apply with the booster for a 1978. There is no adjustment to the rod in the booster.....it is a fixed length. Earlier boosters DID have a rods that your could adjust the round headed screw. But on a 1978...there is not one.

I do not know if you are referring to my post....BUT...I NEVER wrote...telling him to DRIVE that car with the hose removed.....so....hopefully HE is NOT driving the car with the hose removed.

Removing the vacuum hose from the booster for a test is in NO WAY trying to make it seem like or imply that the brakes would react as if they were manual brakes. I do not know how you derived that. You are correct with your statement comparing the two systems...which is obvious...but it does not have any bearing on his problem. Your experience in how hard it was for you to stop your car....you are 100% correct. Literally lifting yourself out of the seat because you are trying to press so hard on the brake pedal to stop your car which seems like it is a freight train...it is generally a booster if it is a power brake system.

THAT is EXACTLY why I asked for him to test it...and NOT DRIVE the car. He will know in about 10-20 feet...if that. Heck..maybe 2 feet. But what the test proves...if the pedal effort is the same with the hose removed...and when it is on...the booster is BAD....in 99% of the cases that I have diagnosed and repaired. I ahve a sense of ;feel;' when I tap the brake pedal and it should cause the car to stop when it is idling and I put it in gear and slightly get it barely rolling. The amount of pedal effort and the 'grab' that I can feel when the front brakes apply tell me that the booster is good and the rest of the system is fine. BECAUSE I ahve had NUMEROUS Corvettes that have brake pedals that take an excessive amount of effort to stop it...which as I wrote....99% of the time...it is the booster....or the check valve in the booster is bad.

DUB

 

From this part right here

maybe there is a miscommunication about the difference between stopping force and pedal effort.

 

Thanks for the info! Seemingly I picked up small brake issue myself while changing pads. Please keep an eye out for that topic

 

...very possible. And as you wrote and I completely agree with is when a booster goes bad....the amount of pedal effort is increased so greatly to try to obtain minimal stopping force. SO...hopefully the test reveal the problem.

DUB