I'm swapping a disc brake rear into a thirdgen that originally had drums. Do I have to change the master cylinder and proportioning valve?

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'88 IROC TPI 355, fully balanced, 9.75:1 compression, fully ported '70 truck heads(for now), Harland Sharp Roller Rockers, Comp Cams TPI cam, fully ported '90 speed density TPI with '86 base so it fits older style heads, Crane AFPR, Flowmaster 3in. exhaust, Edelbrock TES headers, all emissions equipment is still intact, plan on burning my own chips, many more mods and parts on the way as time and money permits =(
Best E.T. with TBI 305: 16.11 @ 82mph
Haven't been to track with new motor
Won bracket races at Numidia Raceway with my '70 Nova
In memory of Dale Earnhardt, I'll miss cheering for that black #3 on raceday!!!

 

Depends on the years. I assume your 88 Iroc has the drums. What year is the disc brake rear from?


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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The rear is out of an 86 Trans Am. And yes, I'm putting it in my 88 IROC.

 

Theoreticly, yes you should change them. There is this little thing called "Brake Balance" that the big three make the brake suppliers work on a lot, big reports, lotsa testing and more reports. When you put rear discs on the back you throw this way off. Will you notice the differnce? Probably not. You will probably never push your car to the point where You will notice. It's mostly on Very low Mu surfaces and sometimes hard braking on spilt Mu.

To answer you question: Should you change the Proportioning valve and M/C?? YES

Do you HAVE to? NO

But if you do change the M/C and P.V. you should also change you Booster. I guarentee the Boost rate for a Disc/Drum is different from a Disc/disc setup.

Just my .02

-Lars