Im having trouble with my front brakes locking up I have 11" rotors with 78 gm calipers on the front with 14" tires 11" rear drum brakes I'm using a 78 ford master cylinder the master cylinder is under the floor I have a 2 psi pressure valve on front with a 10 psi on the rear the adjustable proportional valve i set this up like speedway motors catalog showed
New brake hoses? A little play at the pedal ? Rebuilt calipers are good used? Are the caliper bolts free to move as in not binding? Are the caliper brackets bolted and spaced so the rotor runs in the middle?
Plumbed correctly?,,,large reservoir for the disc small for the drum brakes. Some of the master cylinders have built in residual valves. HRP
Do the front brakes stop the wheels and not release or do they release and are just sliding the front tires immediately? If not releasing ,check the 2# check valve is actually 2#.
I had that problem and the solution was a stronger return spring on the pedal and adjust the plunger rod so it would completely disengage without falling out of the master cylinder.
It sounded to me like he has good front braking but nothing happening in the rear. Maybe jgochneaur can answer joels question.
They slide bet release as soon as I release the brake pedel sory it took so long to answer Ben working a lot
Yep, I agree. He needs larger slave cylinders in the rear [or smaller piston diameter callipers] to get more rear brake bias, then the proportioning valve can be used to limit the rear brakes under extreme braking
You need a metering/proportioning valve. Drums take a bit more initial line pressure to activate than do discs, so this valve holds off pressure to the fronts until the rears get a bite. Most 70's cars have them, and the weight bias of those is such that you can almost always use one straight from the donor car without fear of inappropriate proportioning. Just choose a rear drive front engine car as a donor. Cosmo
I agree with Manyolcars, it just that something got lost in translation.lol. Also I think he mightneed to rebleed brakes and tighten the adjustment on the rears. Also so many good recomendations on this post.YRUHOT...Doug
Nope! Larger rear cylinders will reduce distance [which can be adjusted] but Increase Clamping Pressure which is what he needs. The line pressure is dictated by the M/C and pedal pressure and will remain the same, the larger rear cylinders is how you get more rear brake clamping pressure [or bias]. It is pure fluid mechanics!
As mentioned previously, are the rears adjusted and bled properly? Is the prop valve adjusted properly? Normally, they require 10 or so turns CW to obtain max rear pressure. You stated no history, so is this a new car or new problem? A smaller master bore will result in higher output pressure with a given input force. A smaller wheel cylinder or caliper will result in lower output force for a given input pressure.
This is a new build well about 5 years in the works I have gm 1978 metric caliper with 11" rotors using a 1978 ford granda master cylinder I have a rear end out of a maverick using a 2psi residual valve on the front and a 10psi residual valve on the rear I have adjusted the rear to have a small drag have a good pedel the rear end does have a small slave cylinder
Shorten the push rod. At least that worked for me. Sounds like we have about the same set up. First time out, one block from the house, first stop sign the fronts lock up. Took a while to find the culprit, but once we pulled the rod the brakes loosened. Might give it a try.
The GM metric calliper has a 2.75 bore [5.94 sq in] , Whereas a Granada Calliper which would match the rear end you have install has a 2.6" bore [5.30 sq in ] . so the clamping pressure is significantly greater. Now add to that, the 11 inch rotors vs Grenada 10 inch rotors [255mm] you have increase the brake torque. You really need to match the front to rear brakes [either GM metric rears or Grenada fronts] There is a lot of math involved if you mix and match. Or you than run a "pedal box" with 2 master cylinders and a balance bar
A stock GM metric caliper bore is 2.375", with both 2.0" and 2.75" bores available using aftermarket.
I don't think the push rod is a factor because the fronts are locking up, but not staying locked up. He might need a metering valve.
Probably true , unkledaddy, I was just p***ing along my experience. My fronts also locked but backed off shortly. I have the 2lb, 10lb valve along with the adjustable proportioning valve. Dont know all the math etc, just telling it for my brakes work great after working on the pushrod. Just trying to simplify if possible. I dont know if he got it working or not. Take care , Jerry
Ironically I had the same problem last week, on a Tudor with metrics and 11" drums. Initially the rears were working and the fronts weren't, so I installed an adjustable proportioning valve aft. On the initial test drive with the valve adjusted fully to 'increase', the front discs locked up and squealed the tires whenever the brake pedal was depressed. So I re-bled the rears twice and adjusted the shoes a little tighter. Voila'...............perfect braking now after some minor adjustments to the prop valve.
The Jalopy Journal
