Have a 1988 mustang that has been converted to 5 lug and 4 wheel disc brakes. The rear end is out of a 1994 mustang. The brake pedal is spongy and has a lot of travel before the brakes start working. The system has no air in it. My thought is to install a master cylinder and proportioning valve from a 94 mustang also. What have others done to get the brakes right after rearend swap? Thanks

 

Definitely install an sn-95 booster and master cylinder. The fox booster doesn't have a big enough piston and doesnt push enough/have enough fluid for the bigger calipers. Forewarning, it is hard to get that booster in, it's huge. My car stops on a dime. 100-0 in a few seconds.

 

When I first did the cobra disk brakes with stock mc and booster the brakes were hard with normal pedal travel. I believe a spongy pedal is more than likely air in the system but I could be wrong. I just did the 93 cobra booster and mc. The booster is same size as a sn-95 but with correct bolt pattern and it was fairly simple to install.

 

Simple to install in a fox? Your body must be twisted or crazy deformed. I had to cut the studs off the booster 1/2 inch and cut part of the strut tower to get it to fit. Was impossible with a stock 91 mustang. I got it done.

I'm using the 93+ cobra package straight from FORD. The poster said he had spongey brakes with no air in lines. That is open to controversy because he may not know how to bleed brakes. But the stock fox master cylinder can not provide enough fluid to make 4-wheel discs work properly. He needs to upgrade his master cylinder. You can run it without a booster but that is stupid.

If all is said and done... I doubt he has an adjustable prop valve installed. The stock proportioning valve will NEVER work with 4 wheel discs.

 

From FORD,

This is what you need to do after upgrading brakes.

Most people don't research or care despite spending alot of money.

DISABLE the stock prop valve and install an aftermarket one.

 

My brakes stop from 100-0 in about 3 seconds. My parking brake works as well.

Get cheap and you will complain. Do research, spend the money, and things work like they should.

 

"New proportioning valve" = sn-95 valve. FYI.

Christ I have posted this crap like 50 times.

Learn how to use search. Any technical questions... MESSAGE me... Im bored at work.

 

For the poster's and anyone interested's information.

Fox body vs. SN-95 booster, the bigger one is SN-95 for rear discs.




How the rear discs SHOULD look.


The Sn-95 prop valve...


Forgive me for the dirty engine bay, now it is gloss Deep Jewel Green..... ... but this is where the adjustable proportioning valve NEEDS to be. Mine is FORD racing....but many companies sell this part.

 

My cars not twisted or deformed lol, but yea theres tons of info on this subject most people seem to just elongate the firewall holes. I saw a post by almoststock saying you can bend the studs a little well after a test fit I realized all I have to do is get it in at an angle and not straight in to clear the strut tower so I redrilled the firewall holes at an angle (which is like elongating them I guess) and it went in. The biggest PITA for me was loosening and tightening the bolts under the dash. My booster looks exactly the same btw.

 

Thanks for the help Tony and foxstang. I did a search before posting but must have been to specific to get the help I was looking for. This Mustang isnt mine, it belongs to a customer Im trying to help. He did the swap with 94 brakes/diff, front and rear. Said brakes worked fine before the swap. Nothing else has been changed and has left it with me to figure out. Ive spent 4-5 hours researching since I started the post and have a direction to head now. Ive been a tech for 23 years, 14 years with Ford. Ive done a bunch of Mustang mods. but have not done this mod. Thank you again for the help.