Hey guys,

On all my older, Muscle Car Era cars, I used to have to remove the residual check valve in the master cylinder to run rear disk brakes to prevent rear brakes from dragging. The last time I did this was on a 2nd gen Firebird. I have not done this swap on a 3rd gen until now. By looking at tons of part numbers, it seems that the master cylinders are generally the same for the base model cars regardless of the rear brake setup. The combination or proportioning valve is different by part number on most but that sometimes is because GM wants a different amount of bias on disk rears than drums, from what I have read on the web.

I would like to know if anyone knows if there is such a "residual check valve" in our 3rd gen proportioning valves and if you have removed it for disk brake service. Maybe it needs to be drilled out or it can be popped out like the master cylinders of old...

I am preparing to swap in a 3.23 geared disk brake 9-bolt rear from an 87 Z28 into my 2.73 geared drum brake 89 base model Firebird. I'm on an extreme budget right now and I don't want to spend money rebuilding the rear brakes of a rear end that I want to ditch anyway.

If anyone can shed light on my situation, I would appreciate it.

I am daily driving this car and I don't want to split the braking system apart for experiments and then not get to work on time. I've spent a ton of time reading these forums (and others) but I'm not getting any definitive answers. It seems that some people are swapping with no issues but maybe that's due to worn out springs in the proportioning valve?

 

I I can't contributes much but I heard there is something with the valve on some drum cars to what you mentioned.


Most people say the P valve NEEDS to be swapped but others don't If anything they swap out the internal P valve spring..... Im currently in the process of swapping to LS rears from drum brakes and am in the same boat as far as questions

 

Calculating braking torque - Third Generation F-Body Message Boards

Some good info on the masters and P valve in this thread

 

Only thing I've ever heard of is the prop valve spring getting replaced. there is a good thread around here that covers all the prop valve designs and which ones are compatible - I'll see if I can dig it up. dont recall anything at all on the check valve.

will edit as I find more. this first post indicates you may have a different master cyl than what was used for disk brake cars. might want to look into that to see if it is an area of concern.

https://www.thirdgen.org/forums/brak...ng-torque.html

https://www.thirdgen.org/forums/susp...tml#post887386

https://www.thirdgen.org/forums/brak...tml#post342153

 

Thanks for the help and the links. That stuff plus the factory service manual and all the web searches leads me To believe that setting the rear brake bias is the only difference between prop valves and no difference in master cylinders. Not sure how my setup is going to react but careful road testing seems to be in Order. I'll post results when I get it done.

Thanks again!

 

Yes! Please post results / what you did!

 

Not sure if this helps further but I can say that I have an '87 disk/drum prop, with the granger spring, 4th gen master, and four pot wilwood calipers on all four corners, and it stops as well as my 6th gen. Testing in the air showed that the front wheels did hold before the rears.

 

That DOES help, because I have the Grainger spring upgrade in my 88 V6 disk/drum car and it made a big Difference. If I throw my disk rear into that one first it May work out better for testing. I already have The rear out of that car....

 

Your 4th Gen master cyl leaves an open question, why did you do this, was the '87 master cyl not working for you after you tried the rear disk swap the first time? It also makes me wonder if you even needed the Grainger spring, because I'm assuming the 4th gens all had rear disk, but not certain on that?

 

I replaced the vacuum booster with a hydroboost and wanted the later master cylinder because its a straight 1"(?) rather than stepped (1" 7/8" ??) like the third gen. Also the reservoir on the fourth gen has a round screw cap that I could attach my pressure bleeder, and a low fluid sensor.

 

Ah, all excellent reasons, especially the pressure bleeder! Still wonder if you needed the Grainger spring tho.

 

It's a tough situation, really the prop valve body and valve are the only thing original on the braking system. And it was an all in swap, so I have no idea what would have happened if I didn't change the spring or if I would even know a difference lol. Not something I could even test as the original spring is probably a can of corn by now.