89' RS/blue

Are my brakes going to suck now if i dont get a new master cylinder when swapping from rear drums to a 1LE rear disc 9 Bolt?

ede

porpotioning valve is what matters not the MC and it depends on what you have now.

RB83L69

If you're putting in the PBR calipers (finned aluminum, not the cast-iron ones) then the correct MC for that system is the same one as the drum one anyway.

If you're putting in the cast-iron calipers, where one is in front of the wheel and one is behind, then you WILL need a different MC; the hydraulic requirements (volume and pressure) of that system are considerably different from drums.

LT1guy

iTrader: (1)

I beg to differ on that one...I swapped Redraif's car (87 Firebird V6) to rear discs, using an 84 3.73 disc rear (with the recall kit installed in the calipers), and left the original master cylinder. We did change the prop valve to the 1LE piece, though. I was sceptical as to whether it would work properly with the drum brake MC, but the car stops great, with no issues. Worst case, try it...if it doesn't stop properly, locks up, etc, change to the disc MC.

84 1LE

iTrader: (9)

This is actually correct.The MC is the same for the Rr.drums/ Rr.PBR brakes.The OLD cast iron Rr. calipers use a different MC.The Prop valve is different for each of the 3 types of Rr.brakes.Plus the 1LE set has its own prop valve,but still uses the same MC as the drum/PBR(Rr disc only)brakes.

LT1guy

iTrader: (1)

I'm not disagreeing with what you're saying...you are correct in as far as what the cars came with new. I'm just saying that in my experience, on a particular car that I have worked on and driven, that the MC change wasn't necessary. Honestly, it stops better than my 97 SS, which surprised me with all the horror stories you hear about early rear calipers. I have the correct master cylinder, and expected to have to use it, but tried the stocker anyway since it was polished (and still on the car).

84 1LE

iTrader: (9)

Im not disagreeing with you either.Ive talked to/heard many guys that have done the same as you did & said their brakes worked fine.But if you were to look up the parts in a catalog thats what it says about the MC.Like you said though it may still work with the "wrong"parts.I knowingly drove my 84Z with the "wrong" prop valve for 2 years before finally switching to the correct one for the 1LE set-up.

RB83L69

Exactly; what "works" and what's "right" are sometimes 2 different things.

And, sometimes you can do things that "work" fine seemingly, but never even realize how much better it might be if you did it "right".

Frankly, brakes aren't one of those areas in a car where I'm willing to go with what "works", if I know there's something else that's "right". But that's just me. I'm funny that way; today just isn't a good day to die, for me, and not a good day to kill somebody else, either. Don't think tomorrow will be abetter day for either of those things, or the day after, or the day after that, for a long time to come I hope.

Z28guy83

i didnt swap my MC or prop valve and my brakes work fine and i have the old junk calipers to boot (lifetime warranty from autozone) i would love to get the pbrs but i cant find cores and the parts places want a huge core charge

LT1guy

iTrader: (1)

You imply that anyone who doesn't use the exact part the factory intended is some kind of negligent boob with a death wish; hardly. Brakes are not an area to be taken lightly by any means, but its not voodoo that mere mortals can't tamper with, either. Even GM decided that it wasn't necessary to use a different MC with rear discs, and there is very little difference, if any, between the fluid volume requirements of either style of rear caliper. Many have reported the same results as I have. If you're building a system from scratch, you may as well use the same type MC as the donor car, but the original does work, and work very well; as I said, better than my 97, which has stock brakes with the superior PBR rear calipers, and far better than any other non-1LE stock third gen I have ever driven. To me, the fact that it "works"...and by that I mean works very well,as it should, not just passably...is far more important than what is "right". After all, most of us have piles of inferior "right" parts in our garages, which we replaced with parts that "work".

Justins86bird

Drum and Al rear disc cars got the same mc with a 15/16" bore. Iron rear discs got a different one with a 1" bore. Both will work and both will work right, it'll just have a different pedal feel. I've heard that GM actually switched some 1" bore MCs to the drum one when customers complained the brakes required too much pedal pressure.

89' RS/blue

Alright, thanks everyone for the info. Im just going to get a Wilwood Prop. valve. Thanks for all the usefull info

kevinc

I beg to differ with your differing.

I swapped from drums to J65 rear discs, swapped prop valve to the J65 disc valve, it stopped great. Later that year when I took it for NJ inspections, it failed their brake dyno test bigtime...fronts were great, rears were way low on braking force even tho my pedal was nice 'n firm.

I swapped master cyls to the correct J65 part#, re-bled, and went back to the inspection station...passed the brake dyno easily. Stopping power was even better than before also.

So...if you're only measuring braking by your butt-dyno, you may be missing data.

LT1guy

iTrader: (1)

Did you possibly have some air trapped in the rear lines? I've driven enough F-bodies to know when one is stopping as it should, and this one does. Redraif only knows stop and WFO, with no in between, so the fact that she hasn't hit anything tells me that the brakes are right . We don't have state inspections here, and I have never even seen a brake dyno, but the drastically reduced braking distance and much better feel over the drum setup is all the proof I need. Maybe I'll do a scientific test between both MCs next time I have it off, but I doubt it could be any better than it is right now.

kevinc

It's tough to rule out, other than having bled both calipers in sequence same method for both installations. The pedal was good and firm for both tests, I was surprised at how little force the rears were delivering.

Pablo

iTrader: (1)

i just set up 11" rotors with basically the same calipers in the back as I have in the front on my nine inch rear. I dont have any brake dyno data but the car stops incredibly well and on the jack stands i had no problem stalling out the motor hitting the brake pedal.

I formerly had the drum brakes in the back and thats the only thing i changed, left the prop valve and MC alone.

Car stops great.

BretD 88GTA

This is incorrect. GM dropped the J65 disc/disc MC in 1990 and used the J50 disc/drum/1LE MC in ALL f-bodies.

The J50 also became the standard replacement MC. This is what the dealer would/will install and this is what the auto parts stores carry.

My disc/disc (iron rear calipers) 1988 GTA had a J50 MC. It either came this way from the factory or was installed by a dealer. The brakes worked fine. For my 1LE upgrade I installed a new J50 MC and the correct 1LE prop. valve.

The only real difference betwen the the J65 and J50 was pedal feel. The prop. valve has a far greater impact on how the barkes perform.