I got a 9-Bolt disc brake rear diff for a 86 TA and i want to install it in my 91 GTA
i know that the e-brake cables are going to be different, but is there anything else that will be different from a disc brake diff to a drum diff? i have heard about a valve in the braking system that might need to changed but i dont know

 

you can change the prop valve but you don't have to, i never did in any of the swaps i did. other then that its a direct swap, even the brake lines work. just make sure you get the e-brake cables and the middle piece that the cables hook on to, its a little different from the drum stuff. and enjoy the new found power!

 

Those particular disc brakes aren't likely to ever actually do anything at all anyway, so it probably doesn't matter what else you change, they'll just amount to some extra unsprung dead weight.

Only way they have a prayer of doing anything detectable is to replace the calipers, install the "recall kit" in them, (they don't come with it) put the right combo valve in it, and modify the combo valve. Otherwise they probably won't even be able to hold the wheels still with the car idling in gear on jack stands, they're so useless and ineffective.

 

Combo valve? And how do I modify it?

 

Combo valve = aka proportioning valve

If memory serves, you shim or replace the spring in it. Never done it myself, I've always just avoided that system altogether, so don't take my word for the details.

Of course, you'd need the right valve to start with: the hydraulic requirements of the Saggy calipers are WAY WAY WAY WAY different from drums, therefore the combo valve must be different as well. The drum one will NEVER work right no matter what, gotta get the right one.

So first get the right valve, then search this forum for the mod instructions. I think it's been a few years since they were last posted, because nobody knowledgeable who is actually seriously attempting to "upgrade" brakes, fools with that old crap anymore. Sure, lots of lesser folks just jam em on their cars, thinking "well they're discs they GOTTA be better than drums" .... {insert loud obnoxious "fail" buzzer sound of your choice here} ... but for the most part, people who actually know what they're doing, and would therefore actually know how to do it, don't bother with em in the first place.

And I'm not kidding about them being SO WEEEEEEEEEK that you can put the car on jack stands, crank the motor, drop it in gear, and at idle, they literally can't stop the rear wheels from turning, no matter how hard you stand on em. Again, a search will turn up plenty of complaints.

They say optimum braking percentage is roughly 55-45 to 60-40 on these cars; the disc/drum combo is around 70-30 to 75-25; with those brakes you've got, you'll be DAMN LUCKY to get it up to 99-1, let alone, beat 70-30. "Optimum" is just pie in the sky with those things. Prolly will come out pretty much 100-0 most of the time.

 

The old iron rear disc are not worth the hassle. Find 89-97 rear disc and you'll be miles ahead.

 

you have the disc rear. start with that and make sure the calipers function correctly. if not look to rebuild or upgrade them. search the forum for the prop valve spring replacement. its cheap and easy to try first.

 

I had lots of troubles with my PBR swap on my 9-bolt. Beleive it or not the discs have enough power to lock up your rears even at 60mph!!! It's all about the propvalve! BUT, it' s a really thin line. I did what sofa said by changing the spring in the propvalve to get more brake pressure in the back but even if the rears do not lock up at idle they will lock up when driving!!!

When the car brakes the weight will move to the front lifting up your rear. When this happens the rears will lock up! Not funny unless you are aware of it and know how it will handle but this is definately not for the normal driver.

It took me several rides and hours to find the sweet spot to not lockup the rears when doing a 100% braking. Do it in the rain not on dry streets as the car should behave stable even in wet conditions. I don't drive mine a lot in rain so it's now locking up all 4 tires at the same time in the rain. I can live with that but you have to find your way of liking.

Still my rears will not lock on jackstands when in gear and ideling. I think it's the fault of GM to not install dynamic prop valves like in modern cars where there is a prop valve in the rear which reacts to the load of the car. More load in the back = more braking power for the rears. If the rear end gets light it should reduce the braking power. This static prop valves are crap and maybe ok for racing.