Will i need a new Master Cylinder when swapping from a front disc rear drum setup to four wheel discs, as well what other parts will i need

 

i believe you just need a new proportioning valve

 

I have the same question regarding drum to disk conversion on an 1985 firebird sport coupe. I'm still researching but at the moment i'm inclined to install only a proportioning valve. There is a different master cylinder specified in the partsamerica web site for 4 wheel disk. Both the front and rear bores are a different size. For drums F=24mm, R= 31.6mm. From memory The disk brake bore sizes are both smaller - both around 21mm. So you get a higher pedal to caliper force ratio as you would expect since the effective force on the drum brakes is amplified by the drum pulling the forward show in. When disk brakes are installed you get less pressure on the rear calipers with the drum master cylinder. We'll still need a proportioning valve to drop that pressure even futher to keep the rears from locking up. But it won't be working as hard. I also expect that we'll need to apply more force on the pedal for the same stopping power as before. The master cylinders are cheap rebuilt = $35, but it's the work of putting it in. I like to do things right - but I also do overkill overengineer and spend all my money!

 

GM had a TSB out which basically stated they're interchangeable depending on the driver's preference for pedal effort and travel.

 

In this case, no. Your master is fine. You MAY need to swap out the prop valve but I would test it out first and see how it feels, then make your decision.


Ed

 

I did some research into this when I was going to convert and I found out that you DO need to replace the master cylinder as well as the proportioning valve. The reason for replacing the master is because the port for the rear brakes is slightly smaller than a master for rear drum. Look them up, they are different part numbers.

 

Just because they have different part numbers, and even have different bore sizes doesn't mean you have to replace the master cylinder. I like Aperiron's post: and he backs it up with a TSB reference. (Although no date and number) But the feel of the pedal is what differs and it makes sense that would be so. The 2nd to last post suggests maybe a prop valve is not needed. Hmm - the larger bore size of the drum master cylinder does move the power distribution in right direction. The big thing we're trying to do here is keep the rear wheels from locking up prematurly...

Since screwing around around with all that bubble flare metric plumbing is such a pain I'd rather not fix it till I'm sure it's broke. After I am done with my new axle install I think my test drive will have original m/c and prop valve.

 

https://www.thirdgen.org/techbb2/sho...threadid=58295

 

There's no need to replace the Master Cylinder.

Here's the deal. GM did use two different master cylinders, J50 (for disc/drum AND 1LE disc/disc) & J65 (pre '90 disc/disc - non 1LE). The only real difference between the two was pedal feel.

In 1990 GM dropped the J65 master cylinder and used the J50 on ALL f-bodies. If you walk into an auto parts store and buy a rebuilt MC, it will be a J50. If you go to a GM dealer and request a new MC, they'll sell you a J50.