brakes

Hi All,

I have a problem with brake pedal being low. I installed brakes, new booster master cylinder and brake are still low. I bleed brakes seems like a hundred times. Still low pedal. Can someone please advise me on what I should? I really appreciate any help I can get on this.

A couple of questions:

• did you bench bleed the master cylinder before installing it?
• what sequence have you been bleeding the brakes?  It does make a difference (at least in my experience).

Was the low pedal the reason for replacing all of that stuff, or did this happen after? Master cylinders are different early to late...later cars have a master cylinder piston with a deeper "well" for the pushrod from the booster. Putting a late master on a car that requires the early one will cause this. Iirc, the change was right around '77-'78. 

The pushrod from the booster to the master is sometimes adjustable, as is the actuator rod from the brake pedal to the booster.

Last suspect would be the proportioning valve. If it has been tripped, the pedal will be low, and usually a little mooshy.

Hi,

Yes, I bled the master cylinder then the brakes. First right rear, then left rear,, right front then left front. I did this countless time. Used several bottles of brake fluid. Still the pedal is low. I'm thinking about taking it to a shop. 

It started with hard brake pedal. The brake booster was bad. I replaced it with new brake booster from Corvette Central. Bled it pedal low...replaced Master pedal bled the master cylinder then the whole system still the same..

So this issue(low pedal) is the result of changing the booster, right? You either have the wrong booster, or the pushrod needs to be adjusted, from the booster to the master....possibly the rod from the pedal to the master, also. They are both adjustable to an extent.

As I mentioned before, there are two different depth pistons for different year model cars. If you got a booster for a master that takes the shallow piston, but your actual master has the deep piston, that is the problem. Going the other way the master will not bolt up properly to the booster....

I discarded the old booster. Is there any other way to check to see if I have the correct booster. The reason I purchused the booster from Corvette Central because they said it would be an OEM part.

Unbolt the master from the booster, and pull it away far enough to check the depth of the cavity in the master cylinder piston. Measure from the mating surface(where the master sits flat onto the booster) to the bottom of the cavity. Then, measure the length of the pushrod coming out of the booster, from the tip of the rod to the mating surface(also where the master fits onto the booster). The two measurements should be almost exactly the same. If your pushrod measures noticeably shorter, you have found the problem. You'll either need to get the correct booster, or change the master to fit the booster.

 If the two measurements are close, but still a little short, look to see if the pushrod in the booster has an adjustable end, and make it longer if it does. The idea is to get the pushrod to piston clearance to as close to "0" as possible....there should be VERY little clearance/freeplay, if any. If the pushrod is NOT adjustable, then go up under the dash and see if you can make the rod from the pedal to the booster longer....those rods will always have an adjustment. hth

I thought all boosters for that year were the same. I'll do the measurements this weekend. 

Thank you very much!

The boosters themselves are probably the same, but the pushrods are different. So, unless you used the pushrod from your original booster, this is something that needs to be checked. It is the actual master cylinder piston that determines what pushrod is needed for the two to work properly. I have a new master still in a box that I couldn't use just for this reason...and it is also the reason I know there is a difference to begin with. I checked and found out there were two different master set ups. Just something that does need to be verified, before going any further in the diagnosis. 😁