I have a wilwood 7/8" bore master cylinder feeding disc brakes in front (2 lb. residual valve) and feeding a proportioning valve then a 10 lb. residual valve and finally the slave cylinders in a 69 ford rear end. My problem is that I cannot lock up the rear brakes no matter what I try, the front brakes lock up fine with the proportioning valve closed. I do have immediate pressure at the discharge of the proportioning valve, but I have to push the pedal approx. 2 inches before the slave cylinder moves. In addition I cannot get the emergency brake to hold either, it feels like it is bound up but with the drum off I can move it freely. I should have mentioned all the components are new. I do get some brake grab but cannot get them to lock up. It almost seems like I have the wrong drum but it is an 11 inch drum that is supposed to be the correct drum. Any suggestions would really be welcome, especialy before I go totally nuts!! Thanks, Tom
Try smaller rear wheel cylinders. Oh, and can the prop valve until you get the brakes close with sizing of the calipers/wheel cylinders. Cosmo
Are the drums new? One thing almost nobody pays any attention to on drum brakes is the shoe arc. If you have turned the drums or they are well worn, a new set of shoes may have a very small contact patch. In the old days they used to arc the shoes in, usually by grinding the lining until the shoes fit the drum.
X2... Pull the drums and look at the shoe contact before doing anything else. It will not lock up or hold on E-brake until you get nearly full contact. By the way, smaller bore cylinders would be the wrong way to go...bigger bore would apply more shoe pressure at the same master cyl force.
Many thanks for responding, the drums and the shoes are new but have not been ground to fit, I don't know if anyone does that anymore. I checked the ford manual and see that they offered a 1 inch slave cylinder for police cars but with my light truck (34 ford pick-up) I would think the regular cylinder would be ok. I've thought about pulling the emergency brake as tight as i can get it and driving around to do my own "breaking in" but thought there had to be a better way! I'm still puzzled by why I have to move the pedal approx. 2 inches (pedal ratio is 5:1) before the slave cylinder moves.
Im assuming that you have manual brakes if using a 7/8 bore m.c. Like stated earlier I would get rid of the prop valve and make sure that everything is sized correctly for that bore..... also check the shoes, the primary(shorter shoe) should go towards the front of the car.
is it a dual master cylinder? if it is are is it plumbed correctly? master cylinder have front and back ports. i think the rear brakes if properly bleed and adjusted need time to wear in.
Depending on how mismatched the shoe/drum, it could take thousands of miles to get full contact. Can this be driven on the road? If yes, take it out driving normal stop and go, then feel all 4 brakes for heat. That would tell for sure if the rears are actually applying on normal braking. If they are cool, then something is wrong besides shoe contours.
It is a dual master for manual disc/drums and I've plumbed it the way wilwood said to, small resevoir to the back brakes and large resevoir to the disc's. I did check the shoes and I definetely do not have full contact between the drum and the shoe as probably 50% of the shoe has not been scraped. Sounds like I need to just keep driving it for awhile and adjusting the shoes as they wear in? Thanks, Tom
Thanks everyone, I've said several times before there are some really great people on this board and I know I've learned a lot from you. I'll drive it for awile and see if the brakes get better, it does make sense to me but I think I'll leave the emergency brake alone for now! Thanks, Tom
I've posted this on another thread about brakes but wanted to close this one also. I really appreciated the suggestions I got to fix the problem and tried all of them in addition to a few others without any success. I then thought I might have a kink in the brake line so I pulled the flex line going down to the rear axle in order to get the brake lines out only to discover that the "really neat" flex line I had purchased at a swap meet had a hose opening of less than 1/16th of an inch. I replaced the line with a "plain jane" line that has a 3/16 inch opening and the problem has been corrected. I don't know why I didn't see it when I first built the brake lines but now I really feel like a dumb ----! The brakes lock up great and I will have to re-install the proportioning valve as they actually lock up too fast. Thanks again for the suggestions and I hope this gives someone else something to check for.
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