This was in my last post but I thought it deserved to be its own topic. As always guys thanks for the info.
This was posted last year by Apeiron

Master Cylinder Differences
I just came across this today, and thought it might shed some light on the reasons for the various M/C sizes for those swapping disc and drum rears, or doing the 1LE conversion:

Number: 84-88

Section: 5 (V)

Date: Mar., 1984
Subject: REDUCING BRAKE PEDAL EFFORT ON CAMAROS WITH FOUR WHEEL DISC BRAKES

Model and Year: ALL 1982-84 CAMAROS
TO: ALL CHEVROLET DEALERS


The higher brake pedal effort associated with 1982-84 Camaro cars equipped with four wheel disc brakes may be a source of some customer comments. The J65 brake system was designed with a low pedal ratio for reduced free travel and a firmer and higher effort pedal for the high performance buyer. Owners of J65 equipped Camaro cars not sharing this interest and desiring reduced brake pedal effort can be accommodated by replacing the original production master cylinder assembly with the appropriate J50 master cylinder unit specified below.

MODEL YEAR / ORIGINAL J65 MC / REPLACEMENT J50 MC
1982-83 / 18009370 / 18009367
1984 / 18010574 / 18010572
END
_________________________________________________

Has any tried this, I wondering if changing to the J50 master cyl would offer an improvement. Might explain why my 87 Iroc (disc/drum) stops so much better than my 87 GTA (disc/disc). Of course I would have to use 87 PN master cyl's

 

Don't know if you stumbled over my old messages too, but I put in a 3.73/posi/disc rear from an '84 trans am into my car. I used the J65 master cylinder and correct disc/disc prop valve. After finding out about the recall on the 82-88 rear discs and rebuilding the rear calipers with the $35 kit, the car stopped better than it ever had- including with the 3.42/open/drum axle I had.

So look into rebuilding the rear calipers with the GM recall kit... unless you've done so already.

Oh, found your message; looks like you have! If the brake booster doesn't solve it, look to http://www.brakebleeder.com - I bought the V12 model (W/smart pak and red case) and it pushed a ton of air out of my lines. It's well worth the cash in the aggravation you save! And something tells me that you've been bleeding that brake system as much as I was bleeding mine. When I bought that tool, I nearly cried from happiness because it was so easy. I'll never go back to "push the pedal, release" or bench bleeding a master cylinder "the hard way" again!!

 

How much did you pay for this tool?

 

Thanks TomP
Yup I've already upgraded the rear calipers and currently use a mighty vac for brake bleeding. The power vac booster seems to have passed the test. New master cyl (J50) is only 22.99 at auto zone so it might be worth a try. I also get to play with the brake fluid again (OH JOY). Oh yea I also have a spare (old) J65 which I can return for core and keep the new one I recently installed just in case the J50 route is the wrong way to go. Of course now I have to work on the motivational part and actually do it.

 

Same here the phoenix gizmo is quite excellent
(the video was useful too for me anyway)