I’ve posted a lot of questions on here about different swaps but now I’m actually swapping a 350 in place of my 307 I’m keeping the 200r4 and getting it built to handle the added horsepower. I had bought a 10 bolt 8.5 disc brake axel with a 3.23 gear for a delta 88 I had but now I plan to use it in the 98. My question is do I need to change the master cylinder and proportioning valve? I’ve had people tell me that the 91-96 disc brake and drum brake cars use the same master cylinder.

 

Your info is correct - cars with rear disc brakes and cars with 11" rear drums used the same master cylinder. This is a 1.25" bore M/C. That is NOT the same as the M/C that is currently on your 1984 98, however, which is only a 1.125" bore. Yes, the combo valve is also different. Note that the aftermarket combo valves are not dialed in the way the factory did. GM used a wide variety of combo valves with different calibrations depending on brakes, tires, and vehicle. The aftermarket valves are one-size-fits-none.

 

DOH!

You have to ask yourself, "What will I I accomplish swapping to these parts?"

 

People don't realize that the whole point of the prop valve is to reduce line pressure to the rear brakes so that they don't lock up prematurely. Seems like a waste to go through all the cost and trouble of a rear disc swap just to dial back the pressure so that they provide the same braking force as the original drums. Now, if one upgrades the front brakes to maintain the front/rear balance, and upgrades the tires to provide more ground contact friction, that is a different story, but just changing the rear brakes usually does not provide much benefit.

 

I’m really swapping the axel because I want the gear ratio plus it’s a stronger axel and right now you can see the drum through the 30 spoke wire wheels I have on there
would a master cylinder and proportioning valve from 91-96 car work on my 84 or if you were doing this swap what master cylinder and combo valve would you go with