Enlightenment needed. Is the above-mentioned valve part of a thirdgen combination valve? Or does the "quick take up" design of the master eliminate the need?

JamesC

 

i was under the impression that a residual valve is used when the master is lower than the brakes, to keep the fluid from running back to the master. Not sure if there is a residual valve in the prop valve assembly.

 

I've read that a 2-pound valve is necessary in the situation you mention. A drum system apparently requires 10- or 12-pounds of residual pressure to function correctly. I "guess" the valve--or something that eliminates its need--is in either the combination valve or the master.

JamesC

 

Drum brakes generally have about 5 psi residual pressure and correct on the rear disc being 2 psi.

The stock combination valves in our cars have a metering sectiion hooked to the front brakes (discs) that do not allow the disc brakes to get any pressure til the rear of choice (this being drum brakes ) hits the threshold pressure of 5psi and allows the drum shoes to contact the drums prior to the front disc building pressure. This keeps the fronts and rears biting equally in time frame.

Why? Because the frnt disc pads are ALWAYS in contact with the rotor (for the most part, will not consider pad runoff for this senerio)....BUT, the rear drum brakes are not in contact snce they ride with a gap when released. The time it takes for the shoes to contact is where the metering section allows a slit second build of pressure ONLY for the rears first to build about 5 psi and makje contact prior to all four wheels braking.

Disc/disc will also alow a slight residual of only 2 psi to build a slight contact of rear braking to make sure the car does not yaw on initial bite of all four brakes since the fronts will overpower the rears in stoping force. it keeps the tail settled by just slightly biting first. It is also a longer hydrolic path tot he rears in most cases so the residual also helps conpensate for the delay of rears reacting.

Most of this stuff only works optimum when the car is new. Time deteriorates braking eveness as well as pad and rotor wear differences between wheels.

 

Dean, you may know of the company below because its a California location and because the company is race oriented.

Basically, I want to determine if there's a difference between a thirdgen disc/disc and a disc/drum combination valve, one significant enough to create eventual problems in a disc conversion using the drum valve--as the info below would certainly lead one to believe. At any rate, the links offer some insight into the"general" problem between valves and the company definitely recommends the disc/disc valve on a conversion.

http://www.thebrakeman.com/valvetechi

http://www.thebrakeman.com/valvetechii

JamesC

 

From past experience there should be a different combination valve and/or master cylinder for a disk/drum vs. a disk/disk system.

The purpose of the residual pressure check valve is to maintain a little pressure on the wheel cylinder seals (drum brakes). This prevents the seals from relaxing and thus leaking.

I do not know that a disk caliper requires a residual pressure check valve. I've never known them to. I do know that a drum brake residual pressure check valve on a disk caliper will cause it to drag. Causing slowing of the car, eating fuel, and wearing out the pads.


The metering valve is used to equalize the wear between the front disk pads and the rear brake shoes (disk/drum system). It holds off the application of the front disks until that time that the rear shoes make contact with the drum.

That is all it is good for. As it also delays braking action.

When converting to a disk/drum system to a disk/disk system. Best bet is to eliminate the combo valve. Direct plumbing of both systems. Then once the brakes are bedded/broken in, try them on some hard stops. If the rears tend to lock before the fronts, then a proportioning valve is required.

Note that there are many factors involved as to whether the rears will lock before the fronts or not. Tire friction coefficient, weight distribution, anti-dive, and so on. It may even be found that with full pressure to the rear calipers the rears won't lock under even the highest pedal pressure.

RBob.

 

Note that the '92 FSM has no mention of a residual pressure check valve in either the master cylinder or the combo valve. In a way this makes sense as it has been posted here that the disk/disk and disk/drum systems in '92 use the same master cylinder and same combo valve.

Although it doesn't make sense as I would think that rear drum wheel cylinders would tend to leak.

In the past with iron master cylinders the residual pressure check valve was behind the brass seat for the brake lines. When going with disk brakes it was easy enough to pull the seat, dump the check valve, and press the seat back in.

RBob.

 

The following is from Ed Miller:

The '89-92 MASTERS are the same but the props are indeed different, and the differences are internal to the valve.

According to my Camaro Parts and Illustration Cat, 89-91 shows the same Cylinder, Brk Master, which is GM 18014286. Earlier years show different masters for J50 and J65.

JamesC

 

That sounds better then what has otherwise been posted. A different combo valve would probably mean for a disk/disk system: no metering valve and a different proportional valve calibration.

I did a bunch of googling after my above post and didn't find much on the residual pressure check valves for the years of the 3rd gens. Also searched on quick take-up M/Cs.

Everything I found on GM combo valves of this era showed three functions: metering (to the front), brake light differential switch, and proportioning (to the rear). Of course very few cars of this era had rear disk brakes.

But no residual pressure checks.

And, the quick take-up master cylinders did not show any residual pressure check features. So I have no idea if there is one or not. I know that in the past (cast iron M.C) rebuilt ones started to come without them. Which then had the wheel cylinders leaking within a month.

Not sure what GM is doing now. Maybe a different seal design?

RBob.

 

Rob, The metering section "IS" the pressure check. It will not allow presure to the fronts until the rear pressure comes up to the metering threshold.

Dean

 

Not Eventual problems, IMMEDIATE problems. both my post and brakemans posts say the same basic thing. The drum valve has a higher metering threshold and a lower rear bias.The non-LE disc/disc valve has a lower metering threshold and a higher rear bias, and the 1LE has the same metering the non-1LE has but has an middle rear bias due to larger diameter rear rotors.


I am going off of memory on this info. I believe it to be correct but an writing this only because I am not 100% certain on the valve bias'. I think the drum valve has the lowest rear bias.I someone can confirm of deny this please do and my appologizes if I am mistaken.
The metering portion of my statement is 100% accurate though.

 

A residual pressure check valve maintains line pressure to the wheel cylinder or caliper when the foot is off the brake pedal. It's purpose is different from a metering valve. I listed the purpose of the various pieces in a previous post.

On a 3rd gen with the M/C above the calipers and wheel cylinders only the wheel cylinder circuit would require a residual pressure check valve. Of which there doesn't seem to be one.

And at the same time if there was a residual pressure check for the rear circuit, when disk brakes are swapped onto the rear of the car it would need to be removed. Otherwise the rear disk brakes would drag like crazy.

RBob.