[1stgenbird]

I just picked up a posi disc rear from a late 70's TA to replace the drum non-posi rear currently in my '81 TA.

What parts do I need to do the swap correctly? I have been told the power brake booster is larger for the disc rears and the proportioning valve may need to be changed.

Any guidance would be appreciated.

[drtracer58]

Yes the Proportioning Valve , Master Cylinder is also different , I think I have always seen the small diameter Booster on Rear Disc Cars, I know minw has the smaller Booster

[rexs73gto]

The prop valve needs to be replaced , & it's usually cheaper & a better bet to replace it with an adjustable prop valve that way you can fine tune the front to rear bias. You don't need to replace the booster or master cylinder if you use the adjustable prop valve. The smaller booster is a duel diaphram booster & they use that for room so you don't need to replace that as it fits your car now. With the adjustable prop valve any amount of pressure gained or lost will be adjusted out with it.

[Grand73Am]

The factory setup does include the dual diaphragm booster. It has a smaller diameter, but it's longer, front to back, like fatter that way, so not really "smaller".
Prop valve is different too. And you'd need a master cylinder for rear discs.

This may be something else you'd have to do for the factory conversion. Supposedly the pushrod pin on the brake pedal is in a different location on a pedal for rear discs. Someone posted this pic a few years ago showing where to move the position of the pin to:

[Skip Fix]

Booster is different dual diaphragm 79-80 took a iron MC, 81 a larger register quick take up aluminum MC as the calipers are "low drag". It gives more assist -why you need a lower ratio pedal.

Pedal ratio is different but you can redrill a standard one like the picture. If you don't it will still stop good,but using your little toe for pressure!

Don't worry about a prop valve -T the fronts and add an adjustable in the rear line. That way you can fine tune for different spring rates, shock rates and tires comapred to the factory set one.

Be sure the rear calipers are actually ratcheting with the lever E brake. They often lock up and do not causing poor brake response(too much gap pads to rotor). If you need new ones think about some rebuilt "lifetime warranty" as somewhere they may freeze up. I personally think silicone fluid helps prevent that and I have it in both mine and have no poor pedal feel even on hard braking as some folks say silicone does.

[pontiac1981]

How does the smaller booster work out with rear drum brakes? I have an 81 and I swapped the large booster for the small one, I changed the master and I had to reroute the lines at the master as they were reverse from the original, I left the original prop valve figuring that would work ok. I haven't gotten to the brake pedal rod yet.

[Grand73Am]

You have rear drum brakes now? I think either booster works fine with rear drums.

But I think you must use a disc/drum prop valve, if you have rear drums. The valve for disc/drum is proportioned to prevent the rear drums from locking up before the fronts. So, if you use a disc/disc valve with rear drums, you might find the rear brakes will lock up before the fronts. You could try it as it is, but if the rears are locking up, you'll know what the problem is.

[pontiac1981]

I replaced the prop valve with the same that was on the car, front disc rear drum.

[Skip Fix]

The smaller booster since it is dual diaphragm just gives more assist. So you might need to adjust pedal ratio.

I'd still ditch the stock prop valve and T the fronts and use an adjustable in the rear. Even my factory rear drum setup hard corners the rear would lock up as the suspension unloaded. That way different rear tires, shocks ar when you add the discs you can tune it to your car. Factory prop valve is set for stock springs,shocks tires and an average driver.