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I know this topic must have been discussed before but I can't find what I need to help me fix this problem. The back story. When I got this car it was manual four-wheel drum brakes, not great braking. I bought an MP front wheel disk brake kit with two (four?) piston calipers on each side, 8" dual power booster, dual bowl master cylinder, PV4 proportioning valve, longer pedal and put new lines on the front. Braking improved but I was not yet satisfied. I then put an ebay rear disk setup from a newer '93 - '94 Mustang along with an 8.8 slightly modified rear end so my wheels would clear the rear fenders. The whole thing went belly-up from there. Now I have added a 347 ci engine with 10:1 compression (low engine vacuum) but I love the power. I had to go to a vane style vacuum pump to bring up the vacuum to the power brakes. It seems now that as soon as I touch the pedal I have power assist but no stopping until I put both feet on the pedal and mash it to a slow stop. I still have the original disk/drum MC with the PV4 proportioning valve that came with the MP kit. MP said it should work fine with 4-wheel disks. New lines on the rear. I don't feel that I am getting much brake from the front but I can't really tell. I think this thing should stop like gangbusters but I need some help figuring this out. Any and all ideas will be appreciated. Please help!
 

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Wow. Are you wrenching on this car? Doesn't seem like you even know what you have in parts. Makes it hard for anyone to
give you the correct course of action.
Find out what calipers and rotors. Take a photo and post it.
My opinion is you probably went "backwards" when the rear disc setup was installed. Probably unnecessary to achieve the better braking you want.
You've got a vacuum pump to compensate for the low engine vacuum..... so you have some kind of vacuum with the right numbers in inches of mercury but
are you getting enough "volume" from just the pump to have things work correctly?

If you're not getting enough "brake" in the front, you'll want to know what pressure at the caliper is and what calipers, pads and rotors you have.

Your situation will be a challenge to figure out online...... just a WAG on that.

ex-Global West GM
1991-1995
 
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