Thinpockets

When I put light pressure on the brake pedal it slowly goes to the floor. The pressure on the pedal is like sitting at a stoplight and lightly holding the brakes.

I have already replaced the master cylinder twice (both new from the Chevy dealer) and it is still the same. I have blocked the lines at all 4 wheels and it still slowly goes to the floor. There is no loss of fluid. I can press it to the floor any number of time and it loses no fluid.

What am I missing?

MikeMc71

It really sounds like air in the system, to me.

Thinpockets

We have bled 2 quarts of fluid through and it comes out with no bubbles.

1Z87L8

are they power brakes and can you hear a hissing when pushing on the pedal?
it could be that the booster itself has a leak

Vette Crazzy

if you blocked all 4 lines before the calipers... it has to be the booster or the master cyl. it would'nt suprise me if you need a 3rd m/cyl

vc

Thinpockets

It does have power brakes but I haven't noticed any hissing. Is there a way to test the booster?

ghoastrider1

When My boster went bad, i still had brakes,but was a bear to push.i have seen bosters go bad where the brake fluid gets sucked into the engine,but its rare. Every single time i have had the brake go slowly to the floor,its been bad cups in the master cyl. Speed up the process,and its been bad cups in the wheel cyls...but this last one is usually with some loss of brake fluid. Air in the lines,usually,but not always,results in pulling to one side or the other when applying the "whoa."

SIXFOOTER

Thats the seals in the master cylindar leaking down. Replace the MC

Thinpockets

About a week ago when this started my first thought was to replace the master cylinder which I did. Yesterday when I finished putting the dash together I went for a ride and the brakes are the same. So I put in a second new cylinder - no change. What are the chances of having 3 bad ones - 2 maybe but 3?

About the booster - isn't there a direct connection between pedal and master cylinder and the vacuum booster just adds to the pressure? If this is the case how could a bad booster contribute to the problem?

Tom454

The C3 master cylinders sit on the shelf so long that they rust internally before they are sold.

I disassemble all NEW or rebuilt master cylinders before I install them and inspect the bore for rust.

TimAT

Thinpockets

With the help of you guys and what seemed obvious but hard to believe I've just purchased another master cylinder - this time a Wagner instead of the 2 brand x's - they had no name, just numbers on the white boxes. I will install tomorrow morning.
Any comments on Wagner master cylinders?

JustinGJ

I'll vouch for the MC as well, I had identical symptoms until I replaced mine. Had to bleed at least a quart of brake fluid through before I got any pedal though. Guess I got lucky and only needed one MC though, mine was just a brand X too.

Shark Mann

My MC did the same on my Rodeo. If it continues I would def. check the booster. Did you bench bleed all of your MC's b4 test driving it?!!? or do you have to bench bleed these? Good luck!

-Shark Mann

Thinpockets

Yep, they were bleed before install. I'm really hoping it was just a couple bad MC's. Light pressure on the pedal and it goes down - step on it hard and it stays up.

Tom454

I thought they were Chevy brand...

"I have already replaced the master cylinder twice (both new from the Chevy dealer)"

Regardless of where it comes from, there is a good chance the inside is already rusty.... which ruins the seals as soon as you move the pistons (bleed the MC or step on the pedal).

It takes a lot less time to pull the MC apart and check for rust then it does to install it, bleed it, find out its bad, and remove it.

.01 cents.

Thinpockets

Third new master cylinder - same problem. Pedal still goes to the floor with light pedal pressure. Push hard and it stays up.

Vette Crazzy

Are you bench bleeding the master cyl before installation??
vc

Thinpockets

Yes.

Our new theory is that there may be a problem with the atmospheric valve in the booster. It may open with too little pressure on the pedal. The result of that may be that the booster continues to put pressure on the master cylinder when pedal pressure is calling for no boost. That would be the same as standing on the pedal and forcing it to the floor. Seems kinda far out but I'm getting desperate.

Replacing the booster is not a fun job.

Tom454

That's easy to answer... if the M/C is messing up, the pedal will go to the floor, and you will have no brakes.

If the booster is over-active, the pedal may go to (towards) the floor but the brakes will work & be locked up.