Hey Guys
I did a search and did find similar issues but the answer on how it was solved was never added.

My master cylinder started leaking at the back. I replaced it with the exact same part nr (5460346). The brake booster is fine (new and no brake fluid inside).

Before the leak my car braked straight and it locked up all fours. These get yearly inspected on brake force and I always passed without remarks.
The only thing that bothered me was the pedal travel I had to push it rather deep (not the typical inch and rock hard pedal with the engine off).

With the new master cylinder we get a hard pedal but not rock hard and I can still push it to the floor although it's not 'mushy'. With the engine running / driving I can push the pedal to the floor but the wheels are not locking up. It brakes but it's not safe to go on the road. Emergency stops are definitely not possible.

1 first gravity bled and got the result as mentioned above. Today we flushed all fluid (DOT4) en did the pump and bleed method. It gave exactly the same result.

We don't see a lot of movement in the brake fluid when pushing the pedal and the fluid coming out of the calipers has no air in it.

I assume it's bled in a correct way but could it be that the booster pin (not adjustable) is too short and we don't get full piston travel (leading to not all air removed and soft braking)
Another symptom is a slow returning brake pedal. Maybe there's something wrong with the seals inside the master cylinder?

If anybody has any ideas or has had something similar van we poke your brain please ?

Thanks,
Nick

 

Did you bleed the new master cylinder before installation?

 

agree

Bleeding a new master cylinder, bench bleed, is a must for proper operation...

 

Thanks guys, yes I did but maybe incorrect.
The master cylinder has no bleeder valves and the correct fittings to bend a tube back in to the reservoir is not something you can buy around the corner here.
So I ordered the syringe with the rubber boot (see Ecklers). You fill the syringe with brake fluid, push it hard on to the port and inject. Did this for both ports. I'm sure this method is not as good but I didn't get any air bubbels in the reservoir anymore.
Nick

 

Try this to bench bleed a master cylinder:https://www.napaonline.com/en/p/NDP7001795

 

Any more pointers to look at?
I took the master cylinder off to
A) check the push rod length which is good (not too short, just a tiny bit of play
B) bench bleed it, I saw no air bubbles anymore.
Reassembled everything and started bleeding with the motive pressure bleeder.
In the beginning I had some bubbles at the rear right and front right (bled following the correct sequence) I bled for 2 hours , every caliper multiple times. After the first round I never saw bubbles again. I also pressed the brake pedal with bleeders open thinking that with the pressure bleeding this would get all air out of the MC.
Checked for leaks at every fitting and caliper. No brake fluid anywhere. The motive plate was completely sealed (with a C clamp)
The clevis at the brake pedal is in the correct hole for a PB setup
With engine off the peddle firms up
Started the engine and I'm back where I started off. The pedal almost goes to the floor and I still don't have enough brake force!
I just don't understand it, it was fine before the master started leaking. Could it be a internal defect in the MC.
Any tips please?
Thanks,
Nick
Ps I've got Oring calipers all around

 

Hi

I have the same problem.

Try jacking the rear of car the up so the master cylinder is tilted slightly down in the front. This will probably release some air trapped in the front of the master cylinder.

 

Thank you DS, I'll give that a go coming weekend, It's about the only thing left to do before turning it in to a peddle car
Nick

 

Where did you source the master cylinder? Is it new or rebuilt? Not unheard of to have a bad part. Since everything worked before you changed it out and it isn't working now, I'd say it's a good possibility you have a defective MC.

 

Hi Maj, it's a new one from one of the major parts suppliers (E).
I'll try DS's proposal to get the master cylinder level and if that's not the solution I guess I have no other choice then to take it apart. Maybe one of the seals is flipped .

Nick

 

Something MUST be wrong somewhere...probs the MC. You should not have to really do any special procedure beyond normally bleeding to get the brakes to work correctly if the system is operating correctly. When I bleed the brakes on my 78, I do the normal farthest wheel from the MC to the closest caliper to the MC, a few pumps of the brakes pedal per wheel, and the procedure is done. When I read folks have to bleed the brakes for hours/days, utilizing all kinds of special procedures etc, I chuckle since there is something wrong somewhere requiring this onerous task............

BTW-I still have the OEM MC from 1978 on the car with VBP SS LIP SEAL calipers installed by me in 1985 with SS flex brake hoses at each wheel. I change the brake fluid every 5 years or so with bleeding...car sits 99.99% of the time in the garage...Zero issues!

 

Depending on what year and options You have there are two different piston bores available for C3's. There is also 2 different designs where the Rod pushes the piston. It seems every time somebody has a similar problem to what You have they got the MC from a national parts store and not a Corvette Vendor. Considering where You are I don't know what options You have.
Bottom line, when You have just one Bleeder open, and somebody strokes the MC all the way to the floor, how much fluid do You get ? Do You still have the original MC ?

 

Gale, the bore seems correct I don't think you can miss. If I recall correctly the shallow and deep bore have different push rod diameters. But yes when I open a bleeder and someone pushes the pedal i get a good flow of brake fluid.

I fiddled some more yesterday. Jacked up the car to get the master cilinder level and bled every single caliper again. Not a single air bubble. Pumped the pedal and no bubbles in the master cilinder. Closed the cap, pumped the pedal with engine off and with little movement it became rock hard.

Started the car, pushed the brakes and there it goes to the floor again. When at the floor it barely slows down the car so still unsafe.

I can only guess something is wrong with the MC although I'm puzzled why I get a rock hard pedal without the engine running.

It's getting kinda frustrating and it's going to be 87°F all weekend ... Booo
Nick

 

you can accomplish the = of bench bleed,
by jacking the rear of the car up, until the master cylinder is horizontal,
instead of master cylinder angled up.
then complete the bleed procedure.

 

Remove the front brake line at the M/C and plug the hole.
The pedal should be rock hard.
Replace the front line and plug the rear.
If the pedal is rock hard in both tests the M/C is good.

If you pass both tests the booster may have failed.

 

I didn't see where you mentioned that you bled BOTH bleeders on EACH rear caliper.....
you'll never get good brakes if you only bleed ONE bleeder at each rear caliper!

 

QUESTIONS:

What year is your Corvette??? ( This makes a difference due to the power brake boosters are different)

Were your brakes working as you like them to work BEFORE you noticed the leak at the back of the master cylinder???

And if so....is the power booster the SAME one that allowed you to have good brakes before you noticed the leak???

DUB

 

With out starting the Engine pull the Vacuum Line out of the Booster. If there is not a sudden rush of air heading into the Booster then it is leaking. Or there is no Vacuum source supplying the Booster.

 

Thanks for all the suggestions guys, I really appreciate it. I'd rather try to fix it myself then got to a local shop (there are few in Belgium) which only does so so work..

69Vett, that's what I did the last time I went around with my motive. The rear of the car was jacked up so the MC was level. I couldn't go any higher as my jack was at the end of the stroke, need a bigger one. I saw a YT video in which a guy advised to jack it even higher so all the air goes to the rear reservoir but couldn't achieve that height.

Peterbuilt, I'll see if I can get fittings somewhere and do the test but wouldn't a failed booster give you a hard pedal all the time, making it impossible to push down aka brake?

Sluefoot, yes sir I did both bleeders every time but maybe in reverse order, I did outer first then inner

DUB, it's a restored '74. The booster and MC where replaced and since have driven the car for about 8000 miles. The ZIP item nr's I used (and ZIP confirmed they match) are DB-353 for the BB and DB-336 for the MC. Yes, my brakes where working correctly before I had the rear MC leak. Compared to a modern car I always thought the travel was a bit long, but never to the floor. Near the end all wheels locked up. They are checked yearly at a technical inspection and I always passed on brake force.
No, the BB wasn't replaced, I just ordered a replacement MC from that speedshop I mentioned before. I don't know where they got it but it looked & measured identical to the one I had from ZIP

Gale, I just pulled of the vacuum line (engine not running) and got a rush of air (I last drove or rather tested last Thursday. I also don't hear any hissing with the engine running and pressing the pedal.

I got no issue ordering another MC myself but it takes time and a lot of money on taxes and import duties so I want to make sure everything is ruled out before I press the order button
Nick

 

dembo -

I had the same issue with my 1978, the brake bleed sequence on C3 Vettes is counter intuitive, trust me on this...

Old School bleeding works for me, my daughter pressing the pedal to the floor, me tightening the bleeder screw etc if you do it in this sequence....

Left rear inner
Left rear outer
Right rear inner
Right rear outer
Left front
Right front

I know this sequence seems weird but it works!!