Now if that is a rebuild kit, why does it come with only one piston when there are two in the M/C. Maybe I'll rebuild the original one.

 

Huh?

You are talking about a C3 Corvette here, right?

There's only 1 piston.

 

I have the same problem!!!

This is a copy of a PM that 08vycpe sent me... haven't tried it yet, but it seems to be what we are looking for:

The brake thread is getting kind of out of hand. But to help you with your pedal travel I am assuming that your issue is that you have have no contact with the pedal to booster for a distance, then you feel contact and the brakes work although soggy.

I know for sure there is a rod between the MC and the brake booster and between the pedal and the brake booster.

They are adjustable (as you can see in the AIM) and also come in a couple of different lengths. There is a running post were one CF member states when he received his brake booster rebuild kit he got two different length rods.

Here are some reference pages that show both rods and you can see that there is a screw adjustment of the rod. If the adjustment doesn't take out the dead mechanical air space you might need the different length rod.

What I'm referring to has nothing to do with mushy brakes or excessive travel after the brakes make contact, just the mechanical contact.

1979 Service manual Pg 5-13 Fig. 5.9
1979 Service manual Pg 5-21 Fig. 5.22
1979 Service manual Pg 5-22 Fig. (forgot)

1977 AIM Pg 5G-1 shows both brake booster Y& MC piston rod. You have to remove the MC to see the MC rod.

1977 AIM Pg 5E1 shows MC piston rod.

I can scan and send you a copy if you don't have your AIM or Service Manual.

 

Yes, I recall that thread. It's amazing how few people understand basic hydraulics.

He is correct though that mismatched booster rods can cause a lot of pedal travel as well. Two things however ..

1. If you have the wrong length rod, you CANNOT adjust it out. Don't even bother trying. There is so little "adjustment" down there that it's not even worth fooling with.

2. If you mistakenly have a short booster (pedal side) rod this will become immediately apparent when the pedal doesn't return to it's fully up "rest" position and turns off the brake lights. Or it'll *pop* on its way back up (and down) as the pedal side pushrod actually unseats (and seats) from the booster bladder. TRUST ME on this one because this is exactly what was happening to me.

As I recall, your problem started when you had to slam on the brakes suddenly, correct? If they were fine before that then you had the right parts in there and this is not something you're going adjust out. It's a threaded rod. You're not going to throw it out of adjustment by slamming on the breaks.

 

his problem seems to be the same as mine... and now that you mention "POP" I think I used to here this pop every once in a while but not any more... that is to say when I pushed on the brakes. Baby is in the alignment shop now, did new poly bushings again and hope I don't regret it in 5K miles from squeaking, but when I get her out I will be looking into this posable fix. has this fix worked for anyone else?

 

That still doesn't explain why it only started acting up after you did a panic stop. If you had the wrong size rod(s) in there, you would've noticed immediately after installation.

 

I think there are two pistons in the M/C.
Page 9-5 in Haynes. PG.

 

Oh ok, I see what he meant. It's still 1 piston, there are just 2 stacks to it. I was envisioning parallel pushrods or something.

The rebuild kit has all the seals in it. I think they just don't include the inner piston section because the install of those seals is very simple and easy in comparison.

 

When you find out please post it... it may help me as well, this will be my next big project.

 

I picked up a vacuum bleeder on my way home from work tonight and I'm heading out in the garage to start from scatch....wish me luck.

 

That could just be the way it is. If the pedal is six and a half inches from the floor and there are two inches of free play and the pedal is firm after that than it seems like everything is OK. Had my 74 for 9 years, you have yours for 5 years, when I got this car I felt the pedal was too low, but I'm used to it now. PG.

 

He shouldn't be able to bottom it out though. I can feel engagement after ~1". But I say that with the caveat that my pedal side booster rod is a custom job that allowed me to dial it in precisely.

Please please please before you -> with the vacuum bleeder pop your MC off and make sure you have the right one. If it doesn't look like the rod is seating all the way into the piston, then you very well could have a deep well piston mated to a shallow push rod.

 

Push rod length is ok, started from scratch, re-bled the M/C on the bench, funny thing with that is, I have to push the rod in about half way before fluid comes out the rear port, you would think it would come out of there when you first start pushing it in as it would be the port closest to the firewall. Nothing until half way then I feel it make contact inside, I assume the double piston, front port has more fluid flow than the rear port. is this normal or a bad M/C.

 

Sounds like you've got a bad MC.

 

ordering a new GM M/C, should be in on the weekend, thanx for all the help, will post how I make out....