Packpena

Lost my brakes. To come to a complete stop I have a little pressure on the pedal but after about 1/4 in I lose all pressure and foot goes all the way down to the floor. I’m not able to lock my wheels. Looking at bleeding the brakes, but wondering if its the booster. While checking the master cylinder I noticed a cast piece beneath the master cylinder with on hard line going to it. Wondering what it is. My car is a 67 442 w/ disks up front. Thank u in advance

sysmg

It's called a stop valve. It holds off the rear brakes for a little while so the fronts can engage. I doubt it's the cause. I would rebleed and look for a leak. perhaps between the master and the brake booster.

68442

A leak would be obvious as you would have fluid leaking. You have not said that so i'm going with the Master cylinder being the cause. Rebuild or replacement is in order

Oldsmaniac

Internal leak in master cylinder may be the cause if you are not loosing brake fluid... new master cylinder will fix.

Koda

Busted booster does not diminish braking power, as well.

Packpena

Thank you all for the tips. I do not have any leaks so I will start at master cylinder.

Now because of the stop valve attached to my master cylinder, should I be looking at a drum/drum master cylinder? Or just put a modern disc/drum master cylinder and remove what I have?

Vintage Chief

It's most probably the MC, but I would:

(1) Open the MC reservoir to evaluate fluid level; and,
(2) Thoroughly examine each brake line attachment to wheel caliper for evidence of leaks (remove each wheel, rock each wheel on its spindle forwards/backwards trying to find a leak; and,
(3) Thoroughly examine the MC leaks.

If no leaks found, suspect the MC. As stated, the booster is not likely suspect since the booster only assists in application of foot-to-brake-pedal-to-MC effort (reducing this effort) - the brakes would still function w/o the booster with a great deal more effort, but the brakes would still function.

Don't forget to bench bleed the MC if you find you need to replace/rebuild the MC.

EDIT: You beat me by a couple minutes in posting. If confident of no leaks, as suggested elsewhere in the thread - suspect the MC.

Yellowstatue

Should be a disc/drum master.

Packpena

Disk/drum MC it is.. do I keep the stop valve I have now?
I guess what I should also add is that at the frame I have a proportioning valve with one line connected from from the stop valve and another line connected from the master cylinder.

When I get my new MC will I still need both, proportioning valve at the frame and stop valve?

I’ve had disc/drums before but it was a newer set up with and adjustable prop valve, nonstop valve.

Oldsmaniac

Bench bleed before installing...

68442

Yes

Vintage Chief

I believe you'll find your 1967 disc/drum setup has a brake distribution block mounted to the frame rail. The only thing inside is the differential pressure switch. I believe I'm correct in stating the 1967-1970 disc/drum cars all used the same distribution block as the 1967-1970 drum/drum brake cars. It's not a proportioning valve. I believe the only "proportioning" functionality comes from the "metering" valve (or hold-off, or stop-valve) located under the MC as depicted in your first image. This metering valve is only used in the front brake circuit which (as stated in an earlier post I believe) holds off the front brake circuit long enough for the rear brakes to catch up.

It isn't going to "hurt" anything by retaining the brake distribution block so you might just as well retain it since it's already plumbed into the front brake circuit.

Here's a decent thread from last year:

https://classicoldsmobile.com/forums...-valve-141880/

Packpena

Awesome. Thank you for the clarification and link 🙏🏽. I shall see how it goes.

joe_padavano

That's actually completely backwards. That external valve is a metering valve and the purpose is to delay the application of the FRONT brakes for a fraction of a second to allow the slop in the rear brakes to be taken up. If you look at the lines, the valve is plumbed into the front brakes, not the rear. The metering valve function is incorporated into the combination valve on the 1971-newer cars. Note that this is NOT a proportioning valve. The 1967-70 A-body cars did not use a prop valve, only this metering valve. The factory adjusted the front/rear brake force bias by carefully selecting the wheel cylinder sizes on these cars (disc brake cars used smaller rear wheel cylinders than did drum brake cars for 1967-70).

sysmg

thanks for correcting me.