front/rear balancing four-wheel disc brakes

Message Topic Search Topic Options Post Reply Create New Topic Printable Version Translate Topic

   i've got disc brakes on all corners of my '61 american, that's been heavily modded into a roadster. Scarebird discs up front, a narrowed '98 Mustang 7.5" axle in back, with stock Ford (small) calipers. all of the plumbing is new, a 1" master cyl (manual brakes, no booster), and i've got a 2 psi check valve and Summit knob-adjustable prop valve in the rear circuit. (my master cyl is a couple inches lower than the calipers). i installed the adjustable prop valve back when i had 9 x 2.5" drums up front. at the moment, it's turned all the way to effectively disable it. 
brakes are all working perfectly, but the rears barely actuate, and only at near lock-up of the fronts. parking brake works fine (and stop the car evenly when i've tested them for that).

i need to get more braking on the rears. obviously the rear prop valve is useless. 

should i just plumb the prop valve into the front circuit to get the rears to operate earlier? the thing is, the rear of the car is so light it almost doesn't matter, but i'd like to get it working right.

Author	Message Topic Search Topic Options Post Reply Create New Topic Printable Version Translate Topic
tomj Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jan/27/2010 Location: earth Status: Offline Points: 7555	Post Options Post Reply Quote tomj Report Post Thanks(0) Quote Reply Topic: front/rear balancing four-wheel disc brakes Posted: Nov/29/2016 at 11:06pm
	i've got disc brakes on all corners of my '61 american, that's been heavily modded into a roadster. Scarebird discs up front, a narrowed '98 Mustang 7.5" axle in back, with stock Ford (small) calipers. all of the plumbing is new, a 1" master cyl (manual brakes, no booster), and i've got a 2 psi check valve and Summit knob-adjustable prop valve in the rear circuit. (my master cyl is a couple inches lower than the calipers). i installed the adjustable prop valve back when i had 9 x 2.5" drums up front. at the moment, it's turned all the way to effectively disable it. brakes are all working perfectly, but the rears barely actuate, and only at near lock-up of the fronts. parking brake works fine (and stop the car evenly when i've tested them for that). i need to get more braking on the rears. obviously the rear prop valve is useless. should i just plumb the prop valve into the front circuit to get the rears to operate earlier? the thing is, the rear of the car is so light it almost doesn't matter, but i'd like to get it working right.
	1960 Rambler Super two-door wagon, OHV auto 1961 Roadster American, 195.6 OHV, T5 http://www.ramblerLore.com

farna Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19692	Post Options Post Reply Quote farna Report Post Thanks(0) Quote Reply Posted: Nov/30/2016 at 6:38am
	Typically no prop valve is used on an all disc brake setup. The only thing limiting the rear brakes should be the line size. I've noticed that the splitter fitting on my Jag axle limits fluid volume to the brakes though. Found out because I changed the orientation, and one brake barely gets fluid! That brake was connected to what was originally the line from the MC. Discovered this when bleeding the brakes. So the junction fitting may be limited due to internal sizes. I didn't look at it close enough to see if the passages were all the same, just rotated it back to the original position. You might want to try pulling the three way fitting from the axle and using a plain old brass three way known to be "straight through".
	Frank Swygert

Red Devil Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jul/10/2007 Location: Canada Status: Offline Points: 1743	Post Options Post Reply Quote Red Devil Report Post Thanks(0) Quote Reply Posted: Nov/30/2016 at 11:56pm
	Get a brake pressure gauge and check what you have front and rear. With a tandem equal bore master, should have same pressure front and rear. If not, 1st step is to remove the proportioning valve. Never use a proportioning valve in the front unless it's a special off-road application that needs to limit front braking torque. If pressure is same front & rear and you need more rear brake fit higher CoF pads, larger piston calipers or larger diameter rotors. Generally get better street braking performance and front pad life if rear brakes are sized big enough to need a proportioning valve to limit pressure rise for at-the-limit braking when weight transfer to the front is high. If rears are undersized, tough to optimize overall braking performance. Hope this helps,RD

tomj Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jan/27/2010 Location: earth Status: Offline Points: 7555	Post Options Post Reply Quote tomj Report Post Thanks(0) Quote Reply Posted: Dec/04/2016 at 2:03am
	i have a new-ish 1" bore "chevy" master, two circuits, all new lines, etc. there's no fluid issue, both are getting equal pressure, whatever PSI it is. i don't think there's a "functioning" problem, but a design problem. i could easily swap grippier pads for the rear, they look like semimetallics. but i suspect the rear piston area is simply too small. it's a '98 mustang. 7.5" out of a V6 car. never occurred to me to check into different size caliper bores. is that a thing? lol it's not "fatal", just bad balance. i haven't even done research on what the mustang has, or the Ranger or Celebrity that front donor had, in the rear. i'm actually surprised that i haven't heard this problem discussed anywhere, here or otherwise.
	1960 Rambler Super two-door wagon, OHV auto 1961 Roadster American, 195.6 OHV, T5 http://www.ramblerLore.com

Mopar_guy Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Jun/07/2009 Location: Ohio Status: Offline Points: 4837	Post Options Post Reply Quote Mopar_guy Report Post Thanks(0) Quote Reply Posted: Dec/04/2016 at 8:28am
	I have the opposite situation where my rear calipers are to big so I have an adjustable proportioning valve to reduce the pressure. I would take the valve altogether and see what it does and I would not put that in the front. As Farna mentioned, the line size matters so you might have to increase the line size to the rear. If you still have the same line from the drum brake setup in there, the wheel cylinders don't need as much volume compared to the calipers. Brake line is cheap so that's where I'd start after removing the valve doesn't work.
	"Hemilina" My 1973, 5.7 Hemi swapped Javelin