Well I just finished my ls1 rear disc swap and was wondering if I should change my proportional valve if so do I put an ls1 proportional valve or what? I'm coming for drum btw, and yes I did a search and found jack ****!

 

Found the answer after about 45 mins of searching. 89-92 proportioning valves dont need to be changed, no matter if its drum or disc they have the same valve, didn't find out about the earlier years tho.

 

Can you post that link please? From all that I've read on the net, the drum system in "general" requires a 10- to 12- pound residual pressure which is detrimental to a disc system causing increased pad and rotor wear as well as heat build up which might lead to brake fade and/or lock up.

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. You can't tell just by looking at them. Keep your master and swap the prop if you like, others say you don't have to so you do have an option.

Also, IIRC, the part numbers are different for the two types of valves--I assume to optimize specific brake sytems.

At any rate, I'm still looking for information.

JamesC

 

I'm doing the swap right now too. But I'm going from drum to 91 z28 rear disc. the part numbers are different on the two valves, but what concerns me more is on the disc car there is a wire plugged into the valve and my drum car doesn't have wire. Any guesses?

 

brake light differential switch. Disc or drum, the combination valve should have it.

JamesC

 

James, I just told you in the other post that the 3rd gen brake system does not have a residual valve, it has a metering valve section in the combo valve that bkocks any pressure to the front brakes UNTIL the threshold pressure of either the drum (aprox 5psi line pressure) and the disc type (aprox 2 psi line pressure) They BOTH have metering sections. Neither have residual valves, this is a modern way of making pad contact prior to all brakes biting. Rears need this due to c-clip rear assemblies can cause pad runoff in the rear with rear disc brakes. The disc brake metering section helps decrease this effect and make pad contact first prior to the fronts coming on line and biting.

Dean
ps- metering sections do NOT cause residual line pressure. Older technology used residual valves to overcome drum spring pressure pulling the shoes off the drums and leaving gap, thus a delay of rear brake bite and low brake pedal height.

intead of holding pressure IN the rear of the system, the metering vaklve restricts pressure to the fgront of the system UNTIL the threshold rear pressure is reached so as to aloow for the rear pads or shoes to make contact first. The short take up design of the new master cylinders decrease the low brake pedal height problem earlier master cylinders would causse if using a metering section rather than a residual valve.

 

Thanks, Dean, I noted the other post, which verified my suspicions about the disc/disc and disc/drum combo valves.

JamesC

 

The purpose of the residual pressure check valve is to hold the wheel cylinder seals against the bore (M/C above wheel cylinders such as a 3rd gen). This is to prevent the seals from leaking.

It will NOT hold the shoes against the drum surface. The shoe return springs are too strong for this.

The purpose of the metering valve is to hold off pressure to the front disks UNTIL the rear drum shoes are against the drum surface. At this point the front (disk) brakes get pressure at the same time the rear shoes actuate.

All this does (metering valve) is to even out the wear between the front pads and the rear shoes under light braking conditions. Which for 99% of the drivers this is 80% of the time the brakes are used.

The purpose of a "quick take up" M/C is to quickly put fluid into low drag calipers and move the pads against the rotor. This prevents a low brake pedal. A low drag caliper has the seal grooves cut with more angle toward the pad. This causes the seal to flex out more during brake application and thus retracts the pads more when the foot is off the brake pedal.

The greater retraction of the pads causes less drag for better CAFE ratings. And since the pads are further from the rotor a quick take up M/C is used.

RBob.

 

???

You basically said the same thing I did in both posts. I will assume you quoted me to just agree?
Your labeling is slightly different than I agree with, but close enough for government work. A residual valve holds back, a metering valve holds forward. Both are pressure valves.

 

I found it on here, I'll try and dig it up later. I think it pretty accurate tho, when I had my car up on the rack I drove it in the air and the brakes worked just fine and when I took the car for a drive it works great, if I had 10-12 pond of residual pressure it would at least lock up on the rack costing down form speed.

 

Thanks. There's doubt now whether the thirdgen system was equipped with a residual valve or not. In searching I've found that 89-91 shows the same Cylinder, Brk Master GM 18014286. Earlier years show different masters for J50 and J65. GM, however, pretty much made them interchangeable (All have the 'quick take up' master):

https://www.thirdgen.org/forums/susp...fferences.html

There are different combination valves (if only one was needed, why would GM design valves for different applications?). For exampe:

Valve, Brk Combo (1989 2nd Des) (J50) (1990 [and also 91]) (J41, J42) GM 10136839
J41 - BRAKE SYSTEM, POWER, FRONT DISK, REAR DRUM, CAST IRON
J42 - BRAKE SYSTEM, POWER, FRONT DISK, REAR DRUM, ALUMINUM

Valve, Brk Combo (1989 1st Des) (J65, 1LE, Y82) GM 14089496
J65- BRAKE SYSTEM, POWER, FRONT & REAR DISK
1LE - PERFORMANCE PACKAGE, SHOWROOM STOCK RACING
Y82 - MERCHANDISED PACKAGE, FORMULA

Using the correct application seems to be the right approach to me, but I'm a bit **** in such matters.

JamesC

 

They work fine but I think your right I'm gonna see where I can find one. I dont like to play around with brakes and you seem to know what your talking about.

 

It makes sense that there are two different combo valves, one for disk/disk and one for disk/drum. A drum brake does not require as much line pressure to actuate vs. a disk brake.

This is one reason that my preference is to eliminate the combo valve, run the front disk brakes directly from the M/C (via a T-block if required), and put an adjustable proportioning valve in the line to the rear brakes (disk or drum).

This way the braking system can be properly balanced for the vehicle set up. Changing items such as tires, pad/shoe material, brake parts (calipers/rotors) all change the balance of the braking system.

Now, some may say that by doing this you lose important features of the combo valve. Such as the metering section and differential brake switch. Re-read my post above about the metering valve. All it does is to slow down the actuation of the front brakes. Do you really want that on a high performance vehicle?

And the differential brake switch, all that tells you is what you already know. That the brake pedal is going to the floor because of a leak in the system.

RBob.