[66sprint6]

My son got called in to work last night, and called me about halfway there saying the brakes were failing. The pedal went right to the floor, and wouldn't pump up no matter what we did. Is this a symptom of a bad MC? The MC makes a gushing sound when I listen when my son pushed the pedal.
I'm also having a hard time finding a replacement MC for a '67 A-body with front discs. I can order with a long lead time, but I'll need one quicker if the old one crapped out. What other cars had the same MC? I'd like to order from Rock Auto and get it expedited if I can.

[misterp266]

Try Pirate Jack.

https://piratejack.net/master-cylinders/disc-drum/

[JLMounce]

It can be a symptom of a bad master. It can also be a broken or open bleeder valve or other crack/hole in a line somewhere. I'm assuming you've done a visual inspection of the brake lines to look for leaks at the calipers/drums and at the lines themselves?

You should be able to use almost any GM style master. I'm not sure they really make vehicle specific MC's anymore. Most of what you'll find will be usable in drum/drum, disc/drum or disc/disc configurations without internal holdback valves. This would only really be an issue if your vehicle does not have the factory proportioning valve with the residual built in for the rear drums.

You should be able to source these without wait from any of the major online retailers. They're sometimes called a corvette style master. Make sure to measure the piston size in the current MC and order the same size.

[25stevem]

The Sponsor of this web site Ames performance engineering list what you need in there catalog, in fact two of them!
One with the correct part number and one with no part number at all.

Let’s face it , if it’s the original one then its long long over due that it gets replaced anyway and just be great full that no harm came to the car!

[Scott Thelander]

4 piston disc?
Are either of the reservoirs empty ?
If yes
It’s probably not the master cylinder then if one is low then look behind the tire on the backing plates and look for a wet spot
Don’t turn in your core until you find out it’s not the original one

[77 TRASHCAN]

X2 on post 5!!!

Look for a leak(s).
If no leaks, likely master cylinder.
The master can bleed off internally from one compartment to the other.
Man, a 67 with factory disc, not common!!

[66sprint6]

Thanks for all the pointers and tech. To summarize:

Factory 4 piston calipers.
Brake fluid was down, about 1/4". The bellows in the lid extended a little bit.
No visible leak. No brake fluid on the ground.
Origin of the existing MC unknown. But it looks exactly like the corvette MC, with oval shaped lid, equal bales and the two hold downs at 90* to the unit, 1 above each bale.
It stopped on a dime up until 24 hours ago. We've always been impressed with the stopping ability of this car. It figures, we finally get a good stretch of weather and then this happens.

[Half-Inch Stud]

If water got between the MC and Vac Boost, ice could form overnight and rip the seal between them, if so it might be an easy fix.

Otherwise, top-off fluid and see if/where it goes when you press pedal.

[66sprint6]

After removal

[Scott65]

You could always disassemble and inspect the bore. If it's not scored/pitted hone the bore and put a rebuild kit in it.

[25stevem]

You need to flush out that whole system.
Rust like that floating in the fluid will comprise the needed sealing of the caliper pistons and the rear wheel cylinder seals, and as posted the master cylinder seals and the proportioning valve.

If you can get the brakes working good enough to drive the car safely you might want to take the car to a shop that can pressure flush the whole system which will save you a half a days worth of work and possibly a worn Nee!

[Big Bear]

Check out John Stuart in Stoney Creek. www.johnstuartpowerbrake.com

[Half-Inch Stud]

"mechanical thing wear out because of dirt" -my Dad

[77 TRASHCAN]

That does NOT look like a master cyl from a 60's car, even if it had disc brakes.
From the age of the car, the cyl has likely been replaced, at least once.

It looks more like a Corvette cylinder, although some other cylinders DO look similar to a Corvette cylinder.
I've seen similar cylinders used on different cars, even with front disc/rear drum brakes.
THe 77-80 Corvette cylinder has a 1-1/8" bore. I couldn't tell you if one will work, but I'd try one, if possible.

many of the chain stores will special order parts, and can return them if there's a problem. Most Oreillys can do such. You could order it and bring old cylinder to store, make comparisons to see where you are. Some of the parts stores I deal with don't have such a liberal return policy, and I avoid them IF I'm unsure of anything concerning the part......

[Half-Inch Stud]

My GTO has a G-body Alum MC and works uper, no rust concerns there. Alum is just better than iron for MCs

[25stevem]

That’s all fine & dandy, but water absorption of standard brake fluid can produce rust inside steel brake lines .

Also in terms of MCs cast iron ones experience far less bore expansion with normal under hood temps so they seal better .

[66sprint6]

I ordered a new MC, picked it up this morning, and it's the wrong one. It has the deep hole for the pushrod. And no pin adapter. For manual brakes I believe. So now I'm screwed until Monday. This new one looks exactly the same as the one it replaces, save for the deep hole. Definitely not original. A lot of the purportedly replacement ones look wacky, with a bolt holding the lid down. Never seen that before. Here's hoping that I get the right one on Monday.

[77 TRASHCAN]

Most people f the cylinders I’ve seen with the one bolt cover were for all drum vehicles, and had equal sized reservoirs, front and back. The front disc cars need the larger capacity reservoir.

What is the part number and brand of the cylinder you bought?

[lust4speed]

Originally Posted by 77 TRASHCAN

That does NOT look like a master cyl from a 60's car, even if it had disc brakes.
From the age of the car, the cyl has likely been replaced, at least once...

That looks exactly like my original (and the current replacement) master cylinder, and what was original equipment on my '67 GTO with front disk brakes. I'm the original owner so I can vouch for what was on the car.

My original also went right to the floor when exiting the freeway and caused some excitement. After getting the car stopped with downshifting and emergency brake, the pedal returned to it's normal operating status when I tried it again. Needless to say I still went slowly and directly home and ordered out a replacement unit. It was just like the rubber plungers decided to not go out and make contact with the bore, and then next time decided to work. Stripped it down after installing the replacement and the goofy thing looked like new inside.

[66sprint6]

Dorman M89160 is the one that I received.