Originally Posted by Norm07

I am currently dealing with a brake issue on my 71, 350 automatic, power brake car. I have recently finished a frame off resto, and have replaced the master cylinder (1 1/8 bore, new), power brake booster (new), all new lines, and rebuilt callipers (new pads, resurfaced rotors, no leaks). The brake booster pushrod has been adjusted o spec (using the adjustment tool). I do not see any loss of brake fluid n the reservoir. Motor is the stock 270 hp. I have power bled the brakes a few times and am satisfied I have eliminated any air.

Now on to my issue…..When the car is not running, I get a consistently hard firm pedal that depresses a minimal amount, no sponginess. When the car is running/driving and I use the brakes they engage and stop the car but the pedal almost goes to the floor. It is difficult, almost impossible to lock the brakes up.

Any suggestions as to what could be causing this?
any help would be appreciated.

Originally Posted by doorgunner

(4 corner hoses = flexible hoses that connect each wheel to each brake line)
Buy quality hoses so they don't prematurely collapse.

Yes the flex hoses are new

ALSO...perform a vacuum test on the vacuum hose and the one-way check valve on the brake booster overnight (even though they are new...and double check the small amount of free-play in the pushrod....use your fingertips to slightly press on the brake pedal to make sure there is a tiny bit of movement before the plunger engages the master cylinder piston (your fingers are much more sensitive than your size 12 shoe)
ask me how I know.

Originally Posted by Cool95vette

Could be a bad master cylinder. Just two weeks ago I replaced mine with a brand new AC Delco. It was bad right out of the box. I had to return it to get another.

Originally Posted by 4-vettes

Sounds to me like you still have some trapped air.

Originally Posted by Joey_SS

Originally Posted by HeadsU.P.

Norm,

Just a couple of notes here.
The booster is strictly an "assist" in the brake system.
You either have a firm pedal or you don't. The booster is separate from pedal to the floor issues.
In other words, there is a vacuum system & a hydraulic system.

Sounds like you have a firm pedal, just too low when driving.
So, is the master & booster somewhat of a match? Both Delcos or both sold as a matching aftermarket unit? People get into issues with mix / match.

The M/C was purchased locally and the booster was purchased from Summit Racing. Both are new. Maybe an issue I will keep this in mind.

During your bench bleeding process, did you at any time force the master piston in more than 1 & 3/8 inches? That could damage piston seals, void warranty and allow internal leakage. However, you did state you have a firm pedal with engine off.

I don’t think it was pressed too far….I hope I didn’t press it too far.

Not too likely, possible damage to the master piston. But, how long will the pedal hold constant pressure with engine off? A few seconds? Or well over a minute?
If the pedal moves just a pinch, you have found the issue.

I will check again as to how long the pedal will maintain constant pressure.

The gap between booster rod and master piston: You said it was set at zero.
You want a little gap there. That gap is set at 0.060 or around a 1/16 inch.
Without a gap, the weight of pedal assembly will activate the master. Not much, but enough that the pads will constantly rub the rotor.

I have checked and re-checked this, but will put it on my list to re check .

When you installed the master to the booster and attached the lines, some air was introduced into the lines. But you pressure bled, so you are good to go there.
Seems like faulty brake hoses would show-up in the garage just like on the road.

I have the car on my lift with the wheels off….nothing…bu I will be pulling the Calipers from to further check for leaks…but I am not experiencing any loss of fluid in the reservoir.

At this point, the one thing I would try is the Gravity Method, on the rears anyway. Get two plastic bottles & four clear hoses, let 'er rip. You have nothing to lose.

Yes, another round of bleeding.

Very rare, slim chance, the Proportioning Valve is discounting the rears brakes from doing anything. But the prop valves are pretty reliable.
Good Luck, you will figure it out.

Originally Posted by ctmccloskey

While you are bleeding the brakes you might try tapping on the calipers and the brass unions in the system. I have seen several extra air bubbles out of a previously bled system doing this. Just a light tap is all it takes as it just helps the air break free and get pushed out of the system.

Will try it on the next bleed.

Just so you know I am a proponent of the Phoenix Reverse Bleeder System on my vehicles. The reverse bleeder pushes fluid up from the bottom on the calipers to the master cylinder. First, I remove some of the excess brake fluid inside the master cylinder with a Turkey baster. Then starting at the right rear wheel I push the fresh fluid up TO the master cylinder. When the fresh fluid shows up it will be a slightly different color. This bleeder system allows me to bleed the brakes myself, on the car. Using this bleeder you don't want to bench bleed the master cylinder outside of the car. I use a small bowl and some extra absorbent towels under the master cylinder to assure that I catch all the excess brake fluid. The whole brake bleeding job may take a bit longer the first time but you will be faster as you are familiar

I will consider this, different approach for sure.

There are plenty of optional ways of performing this task but I have found the Phoenix tool the easiest way for me to bleed my brakes, clutches and other hydraulic devices.

The rubber hoses that connect the Chassis to the Calipers at all four corners are frequently a source of problems. We recommend that you replace all four with either new U.S. D.O.T. approved Rubber or SS Braided Rubber brake lines. The braided hoses help keep the rubber lines from "expanding" when the brake pedal is pushed allowing more of the force to get to the actual brake. The old Rubber hoses have a life expectancy of ~15-20 years and then they can collapse and keep the fluid from getting to the brake. The old hoses should be changed for "safety sake" on any 50 year old car or Corvette.

All rubber hoses are new and seem to be working well.

The same goes for the seat belts, the original seat belts will not protect you as designed because the material is shot. The DOT suggests that you replace every part of the belts including hardware. I am going to replace mine as I am not sure just how old the material used is. I would like to survive a 4 G accident without getting broken up physically. That is a random number that Cessna Used for their passengers survive-ability in an accident. I like to exercise my 427 and I keep the brakes at their peak but having me or my passenger get hurt badly in an incident is not part of the equation.

Great Brakes, Good tires, Good Seat belts and the 427 is just beckoning you for a bit of fun.

Originally Posted by Rescue Rogers

IF its bled properly with no issues and good squirt, the gap between the booster pushrod and the master is correct, the master was bled correctly. I would go with a bad booster. IF you replace it return the master and buy it as a set. Mine is from TuffStuff. Make sure you buy a Disc Disc system as they list all of the systems under c3 which include drum/drum and disc/drum. Also you can by from CSSBinc, they have quality stuff as well. Dont buy from your corner parts store ( autozone, oriellys, napa, checker...etc) or a vender if they dont list who made your parts. You cant risk you life for made in Mexico or Chinese brakes....

Originally Posted by Norm07

Yes…..at least twice….as well this M/C has the two bleeder screws on the side of the M/C that we’re utilized.

Thanks Joey_SS for your input.

Originally Posted by cagotzmann

I would try to bleed the MC while on the car.

Here is how I do this procedure.

Get the last bit of air out from the brake master cylinder. - CorvetteForum - Chevrolet Corvette Forum Discussion