garys 68

I just replaced the master cylinder and all brake lines. I bench bled the cylinder, then vacuum bled all brakes. I pushed in the button on the front of the distribution block when I bled the front calipers.
The pedal feels good and solid but the brake light is on (not due to e brake). So I went back and re-bled, but noticed the fronts would gravity bleed even without pushing the button. I pulled and pushed the button and found it would stick slightly in and that it would retract back in slightly after being pulled out, as though it was spring loaded in both directions.
Is that the sign of a bad distribution block?

joe_padavano

First of all, if there's a push button, you have a combination valve, not just a distribution block. I assume you have disk brakes.

Second, the brakes will gravity bleed without pushing the button.

Third, what's happened is that you've pushed the internal piston all the way to one side (common when you bleed brakes) and it may have stuck. This is what contacts the pin on the center connector and illuminates the light on the dash. Unfortunately, you don't know which way it went. The way to recenter the piston is to re-bleed the brakes. It takes a decent pressure differential to move the piston, so you usually have to open the bleeder at one end of the car pretty wide and press the pedal slowly but firmly. Do this with the key in the RUN position and watch for the light to go out. If a bleeder on one end of the car doesn't do it, try one on the other end. Be careful not to push the piston all the way the other way, however.

It's possible that the combo valve is gummed up and the piston may be firmly stuck. It's possible to rebuild them with new O-rings, but you'll need to buy the O-rings individually. I'm not aware of a kit. I just went through this on my 84, and the combo valve started leaking from the center electrical connector, so I just replaced it yesterday.

MDchanic

To elaborate on Joe's (obviously) excellent instructions, the differential switch is the thing that tells you you've got a brake problem.
The idea is that in a dual-"circuit" brake system, if you lose brake fluid or braking ability from one half of the system, the other half will still have enough power to stop you in case of emergency. In our cars (and all American cars, I think), the system halves are front and back. In old VW beetles, they divided it diagonally.

When one half of the system fails, the effect is NOT subtle - the pedal goes much lower before braking begins, you've got much less braking power, and it's much easier to chirp the wheels. That being said, the factory was required to provide a warning light to tell you that half of your brakes are gone, just in case you didn't already notice, hence the differential valve.

The differential valve is a little doohickey that the lines for both halves of the system pass through, with a connecting bore between the two halves. The bore is occupied by a piston that seals it. When the pressure between the two halves of the system is different (ie: when you step on the pedal and one side has no pressure), the piston is driven from the high side to the low side, where it closes a switch and makes the BRAKE light on the dashboard come on.

What you want to do is make the opposite side have no pressure (by opening a bleeder valve), so that the piston goes back into the center.

Before re-opening the brake system, I'd also consider removing the wire from the differential switch - if the light stays on, the problem is probably the parking brake switch.

- Eric

garys 68

Yep, it's a disk/drum setup. No parking brake switch, so I can rule that out.
Thanks for the input, I thought once the center piston stuck at the end of the block you would not be able to bleed brakes from that end.
I was also a little concerned that the button seemed spring loaded in both directions. I though it was push it in then it would return to the end of the bore.
I'll take a stab at loosening a front (or rear) bleed screw, push the pedal, and see if the light goes out. Then bleed it for the 4th time......

garys 68

Well...figured it out. What I thought was spilled brake fluid when I was filling is actually a leaking brand new master cylinder.

Chumley

Well , I'm a little confused now. Is the valve you are all talking about also referred to as the "proportioning valve"? The button on this valve on my car seems to have stuck in the depressed position when I was putting in a new M/C a couple of years ago. According to the repair manual, this button moves the piston in the valve to allow easier flow of fluid when bleeding a wheel cylinder. The car seems to brake okay, but I think I would like to replace the o-rings like Joe mentioned just to have a little peace of mind. Could you just find an o-ring the same size, or does it have to be a "GM" part? I know that new valves are upwards of $100. Chumley

garys 68

The proportioning valve on a 72 does a few things. Earlier years have separate modules, but at least on a 72, they're all integrated into one.
It's got a spring that allows rear braking pressure before front and a differential pressure piston that grounds the brake light bulb in case of front or rear leak.
The button is supposed to be needed for pressure bleeding, not needed for gravity bleeding, but there seems to be some uncertainty about vacuum bleeding. It simply bypasses the spring pressure that proportions front/rear engagement.
If yours works and you dont get a brake light, dont mess with it.
Mine seems to be somewhat spring loaded in both directions, that's where my confusion lies.
If you have to replace it, check out speedway motors, $65 for a new one.

joe_padavano

Once again, DRUM brake cars do NOT have a proportioning valve. All they have is a brass distribution block with the differential pressure switch.

The early (1967-1970) disk brake cars used the same distribution block with the separate metering valve that mounted below the master cylinder. Starting with the 1971 model year, GM combined the functions of the distribution block, differential pressure switch, proportioning valve, and residual pressure valve into a single combination valve to save cost (fewer connections to make, fewer leak points). The cast iron and brass combo valves function exactly the same.

johnspd22

I have some weeping from the rear connection of the prop valve on my '72 olds cutlass. leak is either from brake line connection or larger fitting into the end of cast iron body.

If I take this apart (along with unit for service), what type of o-ring is required for the larger fitting. I am assuming the brake line connection is good, with the other larger fitting being the source of the leak.

I changed from front brake lines last winter and I guess the stress of installation etc. caused leak. Any thoughts? Sorry to jump on this thread, it just looked like it dealt with some of the same issues.

Thanks.

garys 68

Wow, this is old. I wouldn't bother to try to fix it, they're only $65. And brake failure is never a good thing.
If you do want to disassemble, check out the pdf instructions in the link. It has a breakdown.
http://www.speedwaymotors.com/GM-Dis...ets,46424.html

johnspd22

My proportining valve does not match the appearance of this item. This is why I was proposing a rebuild.

Thanks for the feedback.