I understand that there were 2 different master cylinders on 3rd gen cars, the standard, then one with a bigger piston for the high performance rear disk brake package. I am having a hard time finding a part number for the bigger one. I know the prop valves for rear disk and drum brakes are different, but i would like the master cylinder that moves more fluid.

john

 

Why?

Hydraulics is all about leverage. Piston size = the inverse of lever length. In other words, if you use a larger piston in the master cyl and the same size in the slave cyl, you have just decreased your leverage on the parts; the MC piston will move a shorter distance in order to produce the same results at the slave, and will require more effort but less pedal travel at the master to produce the same results at the slave. Nothing else changes.

However, the sizes of the 2 pistons in the MC are sized such that they apply the correct pressure and flow to both the fronts and the rears. If you change just one piston, you upset that balance.

In point of fact, the MC for the PBR rear disk system and the drum system are the same one, except for 1LE cars (different front brakes). The MC for the Saginaw brake system is different, because the slave cylinder at the wheels is sized differently and therefore has different hydraulic requirements. The best thing you can do is to use the correct MC for the brake system you actually have installed on your car.

 

front set up...Stock disk brakes

Rear set up... LT1 rear factory disk brakes off a 94 or 95

My master cylinder is the stock front disk rear drum for my 89.

I was looking at teh fact that it will take more fluid to move my new rear calipers than it did my drum wheel cylinders, there fore i thought i will need a bigger pistoned master cylinder to move more fluid to make it work properly.

john

 

That rear has PBR calipers, just like the 89-up third gen ones. Their hydraulic requirements are about the same as drums.

Looks to me like you already have the right MC. You might need a different prop valve.

 

really, that boggles me considering the size of the piston in a caliper verses a wheel cylinder, but the calipers are pretty small.
well, i guess i will be giving wilwood a call for an adj prop valve. or should i just yank a prop valve off a junk yard third gen that has rear disk brakes???

john

 

Well, all you have to do is go to NAPA or PartsAmerica or wherever and look up the part number for a master cyl; it's the same for all of them 89-92 except 1LE.

Personally I wouldn't fool around with a used PV. Besides which, finding a 89-92 (can't be 88-back) rear wheel disk car just laying around in the junkyard, is a fairly low-probability proposition.

 

i got 2 rear wheel disk third gens for my picking in my local salvage yard. DOn't know there years though. I was just looking that i wouldn't have to deal with the fabrication aspect of the brake lines if i went factory proportioning valve.

john

 

The years definitely matter. The PV for a car with Saginaw rear brakes (88-back) is different and won't give you the results you're looking for. Needs to be 89-up.

 

Well, check out this Chevrolet Dealer Service Bulletin:

Buelletin # 84-88
Section 5 (V)
Date March 1984

Auggie

 

It's the same for all of them inlcuding 1LE. Up until '90, GM used (2) master cylinders - the J65 for disc/disc cars and the J50 for disc/drum AND 1LE.

Starting in 1990, GM dropped the J65 MC and striclty used the J50 in ALL F-bodies - disc/disc and disc/drum. The J50 is the standard replacement MC regardless of your factory brake configuration. If you go to the dealer and ask for a new MC, you will get a J50. If you go the the local auto parts store, you will get a J50.

The J50 is the MC you want, and from your description, is what you most likley already have in the car. What you need to change is the prop. valve. I'd go with the correct 1LE valve for your year OR you can ditch the factory PV and go with a Wilwood adjustable unit.

Stainless Steel Brakes Corp. also offers adjustable PVs and has a mdoel that includes a pressure gauge. Personally, if you decide to go with an adjustable PV, the SSBC w/gauge would be my choice.

 

Yup, all that info is right on the money. The J65 M/C was designed with a low pedal ratio for reduced free travel and a firmer and higher effort pedal for the high performance buyer. Worst brakes I ever had on a car. The bulletin I listed above said that people hated the feel of them brakes so they made avaiable the J50 M/C.

Auggie

 

where do i see that bulletin at?

it isn't showing up on my comp

 

If you upgrade to larger aftermarket brakes, the pedal will become more touchy and loose feel. You'll want the J65 larger bore. I swithed mine and am much happier.

The people that complained about "more effort needed to press the pedal" were old ladies comuting daily and wanted that easy cush feel.

 

Frogmanjohn, If you want a copy I could e-mail it to you.

Auggie