RMJ11949

Sorry if this has been covered before.

1980 Coupe 350 4spd

1) Brake pedal goes directly to floor with motor running.
2) With motor off it pumps up firm.
3) Engine vacuum perfect and steady.
4) No fluid leaks.
5) With foot on brake (after pumping) and engine off, the pedal drops slightly when motor is turned on.
6) With Booster vacuum line disconnected and plugged car brake fine.
7) Changed master cylinder and booster check valve and filter.

I Suspect ballooning of one of the brake hoses but lost my wheel lock key and can't pull wheels yet. My next guess would be the booster, but everything I read says I should have a harder time braking or a problem with my engine idle.
If anyone has had this experience please chime in. I am really not looking forward to removing that booster if I don't have to.
Thanks!

Sluefoot

I'd say yer gonna hafta pull the vacuum booster.....Had a Blazer that did the same exact thing....NEW booster.....problem gone!

DUB

Honestly it sounds like to me you have air in the brake system, You need to bleed it out again....and hopefully you 'bench bled' the master cylinder before you installed it. AND when you bled the system you got the car in a position so the master cylinder is basically LEVEL.....so the rear of the car needs to be raised quite a bit.....and depending on the method you used to bleed the system,...it could require MORE bleeding of the system.

AND hopefully you carefully checked to make sure the master cylinder is correct for the type of booster.

Your booster is fine from what you described..

DUB

DUB

If he does and it works this would amaze me. It would not be the first time. From what he wrote...the booster is doing what it should.

A vacuum booster with NO vacuum...the pedal is HARD.

Pump up the brakes and hold down on the pedal and crank the engine...the pedal goes down...as it should.

I still stand behind air being in the system.

DUB

RMJ11949

Thanks to all who replied. I just found the key for my wheels and will be examining the lines and bleeding the system. I did a good job bleeding the master and my pedal (w/o) the engine on pumped up hard. I couldn't find any faults with my old master (which was about 5 years old). I tried the booster test, held the pedal down and started the engine. It did drop, but then went all the way. I tried pumping on the booster with a vacuum pump and the pedal behaved the same way. This all started after starting the car and trying to move the car, the brakes worked for a few times then I felt a clunk and the pedal went all the way down. Booster spring? But the pedal returns up. Saturday I'll put the car on blocks and give it a good exam. I hope it's not the booster, I'm 6'1", 215 lbs and 63. I fit under the dash 20 years ago and it was not fun then. i'd rather do my own work. I will post my results for the benefit of others that might have the same symptoms. Many thanks!

DUB

Give the bleed another try and I HOPE that it comes out OK. Please try not to forget to get the rear of the car up so the master cylinder is as level as possible. SO...'putting the car up on blocks' just won't work out well when you are bleeding it...because the master cylinder will not be level. That will be fine to do an inspection for leaks.

Not meaning to insult your intelligence...but when I bleed out an entire brake system. I put a floor jack under the rear differential spring mount area and raise the car up as far as it takes to get the master cylinder level. I then slide a jack stand under each frame rail IN FRONT of the rear wheel and let them pinch to the frame...at the same time LEAVING the floor jack right where it is. I bleed the rears and while it is still up in the air on the floor jack...I turn the steering wheel and bleed the right front caliper and then turn the wheel the other direction and bleed out the left caliper.

I still feel the booster is fine. It does not take that much air to get the pedal to fall down. AND...bleeding a brake system is much easier than swapping out a booster.

I assume you are bleeding the system by the manual 'pump the pedal---hold it down---open the bleeder----tighten the bleeder---release off the brake pedal'' method????

I use a pressure bleeder that forces fluid through the system. And I can say that if a master cylinder is replaced...even with my pressure bleeder...it takes several seconds for all of the air to get pushed out of the bleeder....so....if doing it by the manual method. It could take SEVERAL bleeding steps to get the air from the master cylinder to come out. Stop and think about how much fluid is actually coming out when you open the bleeder and then imagine on how much of the line that fluid actually took up in the line.

DUB

RMJ11949

Hi,
Thank you for your reply. The car is on blocks, master cylinder is level. I vacuum bled (hand pump) the system several times. Next, I bled it the old fashion way with a helper. It did make a slight improvement with the engine off but once started the pedal goes almost all the way down. I cleaned and greased the threads of the bleeders, did everything by the book. I've owned this car since 1985 and did this many times before. I agree that it should not be the Booster, but what is left?The thing that bothers me is how this all started, upon pressing the pedal for the first time with the motor running I heard and felt a pop, as if I were standing on a cardboard box and it just let go. What are your thoughts?i'm ready to order a booster.
Thanks again,
Rick
QUOTE=DUB;1590470877]Give the bleed another try and I HOPE that it comes out OK. Please try not to forget to get the rear of the car up so the master cylinder is as level as possible. SO...'putting the car up on blocks' just won't work out well when you are bleeding it...because the master cylinder will not be level. That will be fine to do an inspection for leaks.

Not meaning to insult your intelligence...but when I bleed out an entire brake system. I put a floor jack under the rear differential spring mount area and raise the car up as far as it takes to get the master cylinder level. I then slide a jack stand under each frame rail IN FRONT of the rear wheel and let them pinch to the frame...at the same time LEAVING the floor jack right where it is. I bleed the rears and while it is still up in the air on the floor jack...I turn the steering wheel and bleed the right front caliper and then turn the wheel the other direction and bleed out the left caliper.

I still feel the booster is fine. It does not take that much air to get the pedal to fall down. AND...bleeding a brake system is much easier than swapping out a booster.

I assume you are bleeding the system by the manual 'pump the pedal---hold it down---open the bleeder----tighten the bleeder---release off the brake pedal'' method????

I use a pressure bleeder that forces fluid through the system. And I can say that if a master cylinder is replaced...even with my pressure bleeder...it takes several seconds for all of the air to get pushed out of the bleeder....so....if doing it by the manual method. It could take SEVERAL bleeding steps to get the air from the master cylinder to come out. Stop and think about how much fluid is actually coming out when you open the bleeder and then imagine on how much of the line that fluid actually took up in the line.

DUB[/QUOTE]

DUB

If you are NOT loosing brake fluid anywhere.

I would then have to concede defeat... and then advise replacing the booster....or at least verify that your brake pedal assembly and related parts are still in good condition. It does not make sense because the booster is reacting as it should.

DUB

Peterbuilt

The rear calipers have two bleeders, an inner and outer, did you bleed the inner then the outer?

DUB

GREAT POINT...I often times miss the obvious....assuming that it is a known by all of those who own a 1965-1982 Corvette.

DUB:

RMJ11949

I bled all 6 bleeders and had a bubble free stream of clean fluid.there are no leaks, lines and hoses are in good shape, new master cylinder ( have a feeling old one was OK). Engine vacuum good, new check valve, hose.i'm going to order a Booster and bite the bullet.
Thanks....I'll keep you posted.

RMJ11949

Hi,
Is there any chance a caliper can leak air in but not leak any fluid out? How would I know? I ordered a booster but it still does not make any sense, a bad booster should give me a hard pedal. This is driving me nuts.
thanks!

DUB

YES it can....but...it can not do it UNTIL you drive it. SO...sitting their in your shop...NO...the caliper can not suck in air...because the wheel and rotors are not turning.

BUT.....200 miles down the road and your 'BRAKE' warning light comes on.....and when you bleed it you find air.....then YES....the rotor run-out and/or bearings are NOT within tolerances.

The only other thing that I can think of is that the master cylinder may be bad. I have a brake pressure testing gauge where I can connect it to the front or rear brake and watch how much pressure the system is supplying when I press the pedal. BUT....I have only come across a very few NEW master cylinders that when I bench bled them...they did not act right when i was bench bleeding them. I do use rebuilt master cylinders where the coding on them is so that they are factory correct.

Just throwing out thoughts....

DUB

Peterbuilt

RMJ, you did mention that you had your Corvette for a number of years but when you mentioned the lost key for the wheel locks I had to mention 6 bleeders. Now I understand that you knew that already.

DUB, Not having a gauge that can read the high pressures generated by the M/C, would removing and plugging one brake line at a time and getting a rock hard pedal prove if the M/C was good on both sides?

DUB

GOOD QUESTION...I honestly do not know the answer to this scenario. I guess it could provide some insight for a DIY'er who may not have a gauge.

My concern is the 'POP' sound he got when he pressed the pedal and it went down hill from there.

DUB

RMJ11949

Hi and thanks again,
The car never left my garage, the pop sound started as I attempted to back it out. It had not been driven for a year but run periodically. I realize its possible to buy a bad master cylinder (this one was new, not rebuilt) and tested well on the bench. But I wonder now if the master cylinder that was in the car was bad, it was less than 10 years old. BTW since I've owned the car I've changed calipers, hoses and have bled this many times but never had this problem. I want to suspect a bad caliper, but nothing is leaking. I don't dare drive even a 1/8 mile until I have a better feeling pedal. I have a new booster on order but I'm going to bleed the system at least two more times (I can always return the booster).
With the booster disconnected shouldn't my brakes work unassisted with a hard pedal?

gungatim

the pop sound got me too. I would inspect the linkage, and since you replaced the M/C recently, that is where I would check, maybe something goofy with the rod/plunger, alignment, etc.

can you have someone id exactly where the pop is coming from while it happens?

Tampa Jerry

Just a thought here, all of the bleeders should be on the top of all calipers. If any are on the bottom, you will never be able to bleed the system. Jerry

RMJ11949

Hi & thanks,
The "pop" sound only occurred once when I first hit the brake when in reverse. Since that moment the pedal always goes down with the engine turned on. The linkage is fine. Someone asked if the bleeders were on top, yes they are. If I could only get a leak I'd replace the part and be on my way. I'm bleeding the system again this weekend.
In the mean time I'm enjoying my '08.

RMJ11949

Hi,
Since the "pop" only happened once and since I did not expect it I was not paying attention. It seemed like the "pop" was coming from the front of the car not the rear. Which is why I changed the MC.
It felt like I broke something.