Not to be a post whore, but I hope to get some more input with my rear brake dilemma in the '52 DeSoto wagon. The details, from my other thread are, '03 Sport Trac disk brake rear, new, correct this time, rotors, calipers and pads. All new lines, hard and rubber. Speedway dual 7' booster, on the firewall, dual chamber MC, (I think it's a 1" bore but it's so long ago since I bought it I honestly can't remember), and a pedal and bracket from a Dodge Challenger. I had no rear brakes, discovered I had the wrong rotors and calipers, which I replaced yesterday with no improvement. I changed the pedal ratio from the 3.5:1 to 4.5:1 today, which is all I can get by moving the pushrod mount up the pedal (any more and the angle looks scary). That made a visible difference, the rear wheels will now slow with the car off the ground, they wouldn't before, but they can't be stopped against the engine in gear. On the ground, the front brakes hold, but it's definitely "pushing". Checking the pressure just now, I have 400 PSI at the rear. I didn't check the front, as they seem to work, but I certainly can. I can't find specs for this on-line, but seems like it should be the same as the front pressure, which is supposed to be 1200 or thereabouts from what I see. Bad booster? Still not enough leverage on the pedal? I'm open to any suggestions at this point and have come to the conclusion that I have a faulty component, booster or distribution block, which is for a disk/disk system by the way.
Not enough pressure. Pedal ratio plays a big part in making enough pressure. It's also possible the prop valve isn't compatible or the master cylinder bore is too large for your setup.
Starting with the basics, 1) are there any leaks in the system? 2) do you have a proportioning valve? If so, what happens when you turn it full bore one way, then test, then the other way, then test?
I think you need to raise the pedal ratio closer to 6 to 1. had a similar problem with an old Chevy tk with a similar M/C and booster and that's what it took to get it to stop. Good Luck
Your measured 400psi will only increase to about 533 with a pedal ratio increase to 6:1 from 4.5: 1. 400 / 4.5 × 6. Obviously more and a large step in the right direction , but enough?? Whats the master diameter, 1" or 1 1/8? Booster looks like 8" double, right? Chris Sent from my SM-T515 using The H.A.M.B. mobile app
I put one of those double 8" booster/Corvette style master cylinders on a car one time, I had nothing but problems with it. When I down graded to a Dodge Aspen master cylinder and booster, I had brakes. Those 8" boosters and master cylinder kits are junk. Gene
My post hasn't been deleted, but the master cylinder, booster, prop valve and pedal assembly ARE being deleted.
1 1/8", and it's a 7" booster. I have NO room on the firewall, the hood hinge bracket will have to be trimmed to get an 8" in.
"My post hasn't been deleted, but the master cylinder, booster, prop valve and pedal assembly ARE being deleted". Sorry I was unclear .... I deleted what I myself had written in the two posts above as I had read on another of your posts that you were eliminating the Challenger pedal assembly. I changed it to avoid any more confusion. My posts were regarding the Challenger pedal assembly which is soon to be eliminated from the project and therefore no more discussion is needed regarding it.
My advice, see if you can source the master, booster, distribution block and pedal assembly from the vehicle (or type of vehicle) your rear end is from. I'd be interested to know the pedal ratio on Explorer P-up.
I'm thinking the master bore is to large = not enough pressure. I'm running a 1" master and discs on all four wheels (8.8 rear and GM metrics front) without a booster....my heap stops OK.
That’s exactly the set up I have, and have 1” MC coming. Thanks for the report. Sent from my iPhone using H.A.M.B.
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