I have my do it yourself 4 wheel disc brake conversion bolted in and ready to make the steel lines. The master cylinder is 8 inches higher than the calipers when the truck is level. The whole braking system is a transplant from 1 vehicle so these parts all worked together in the past except it was abs and I chucked that whole abs setup. My question is do I need to run residual pressure valves with the master cylinder now under the floor. It was up on the firewall in its original setup. The 4 calipers master cylinder and booster are second gen ford explorer. Thanks
If the master cylinder is higher than the calipers, I wouldn't bother with RPV's. You'll already have bit of "head pressure"
Trying to picture a MC under the floor and the calipers 8 inches lower. Sounds like a Power Wagon stance... I'd put them in to make sure, especially if it's going to be driven in conditions where a PW could go. It's often a deal where you need more than one line to reach front and back, so another junction is no big deal.
No! The best method if possible is to mount Remote Reservoirs higher up and trash the residual valves. Race cars have been doing this method for years. But your vehicle already has the M/C 8" higher than the Callipers, Just make sure the Reservoir and the Cap is Leak Free/Air Tight then add a Vent/Breather tube that goes higher up
Posting this so if in the future a search turns this up. After many hours of research I find the consensus from all the big after market brake kit makers is run a 2 pound residual pressure valve on disc brakes and a 10 pound residual pressure valve on drum brakes no matter where your master cylinder is located. Valves should be close as practical to the master cylinder.
^I've read this too and that's how I do them all. My heap has the master under the floor with residual valves added and works fine.
In truth you don't notice the difference in most rigs with or without a residual valve if you drive them every day and never let them sit. It's when your rig sits for several days at a time between being driven that the reason you need residual valves shows up. As in UH I let it sit for two weeks and now I have a mushy pedal and have to bleed the brakes. The residual valves maintain pressure against the cups in wheel cylinders and the seal rings in calpers. Without that pressure it's going to draw a bit of air in over time. The old single piston master cylinders have a residual (check) valve in the cylinder that took care of that issue.
Uhhh….nope, the residual valve for drum brakes is simply to keep the air from entering upon brake release, when the return springs snap the piston back it creates a low pressure that can cause the cup seal to collapse slightly and allow air. 2 lb residual valves on disk breaks is to prevent fluid from running back to the master if it’s lower then the caliper. Neither have anything to do with air magically getting in while sitting.
For 24 years my truck hasn't had residual valves with the master under the floor. It just slightly higher than the calipers, pedal pressure seems fine to me.
Never have used residual valves with any brake system I built where the master was higher than the calipers or wheel cylinders. Even sitting for 5-6 months over the winter they never lost a full pedal.
I forgot to mention that residual valves in drum brakes were replaced with cup expanders in the wheel cylinders…
Yeah... That's not at all the reason for residual pressure valves. The reason they are used is to prevent backflow to the master so you don't need to pump the brake pedal every time you apply the brakes. If you have air leaking past the seals when the car is sitting, you need to replace those seals. Do you need to bleed the brakes after your car sits over winter? Sure don't.
Just a way to extract a few more bucks from you... Yes, if the master is lower than the calipers you need them, otherwise no. Just like those useless brass 'proportioning' valves they all insist you need.
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