Residual valve needed or not?

Currently repairing one of my leaky brake lines on Sinister Sally. I originally installed a one inch single remote master like this.

Then I split the line with a tee and used 2 of these 2PSI valves-

One between the master and the front disks and one between the master and rear disks. Do these valves really make a difference or not?

 

The theory is to use them if the master is lower than the wheel cylinders. I’ve heard mixed reviews.

 

I think Spirit uses them on the front to limit lock up. I don’t go along with the theory of needing to control flow when master is lower than wheel cylinders. I think air bind takes care of that.

 

Just something that's to make money for the suppliers, I never use them....OLF is correct about the air bind in the system.......

 

Not sure, but in a closed hydraulic brake system, the fluid is not going to "run downhill" back to the MC. I don't use residual valves, but I can see how one might help with the relatively long pedal stroke needed to push fluid to the front discs. Mine does fine without it, but . . . it might help in some systems.

 

You have a pressurized closed system the ONLY way for fluid to flow backward is if you let air in at the far end then it will run back or out of the system and without the air therefore you have an air bound system.. That is the way I understand the system....

 

Arn,t old enough to know about such.

 

Ever put a straw in a glass of water put your finger on the top and lift the straw up and marvel at the water straws in the straw?

 

I think there's been a tendency to over simplify here, at least a bit . . . .

What's not being factored in is the wear on the break pads, (this is really a disk brake specific issue) increasing piston travel and therefore the volume of the piston bore , which means more fluid has to be transferred to the piston bore over time as the brakes are applied.

If the master isn't kept full, at some point there may be enough air space in the master cylinder that enough fluid can drain back from the caliper that a single press of the pedal will not move enough fluid to actually apply the brake.

As long as you keep the MC at the recommended fill level, there won't be any issues, but if you don't keep it up to compensate for increased caliper bore volume resulting from pad wear, you could have a problem where you'll have to pump the pedal to get good braking.

 

"If the master isn't kept full, at some point there may be enough air space in the master cylinder that enough fluid can drain back from the caliper that a single press of the pedal will not move enough fluid to actually apply the brake"

Actually that "air space" is a vacuum per se (it's less pressure than atmospheric pressure) granted some MCs have bellows to take up the vacuum and the calibers can/should move in and out, it is a closed system but both ends can move. If you use your car regularly, you would probably wouldn't notice anything, but when you take it out from winter hibernation you MIGHT notice a soft/low pedal.

When you run your brake lines, try to leave a 6" straight section open, try it without a RV, if you get a soft pedal, install one.

"Parts left out don't cost anything and they don't have any maintenance problems"

 

Sort of, but not over a long period of time as the pads wear.

With new pads which have nominally .300" thick friction material, while new there's maybe .050" movement of the piston from fully retracted to apply, . . . . . while at 2/3 worn down, the piston would have to travel .450" from a fully retracted position.

That's a large change in system volume and you'd have to compare it to how much fluid your MC moves in a single pedal press.

Most "T" owners are maintenance savvy and keep up with checking fluids, but for those that aren't and drive a lot, they;re the ones that could go to back the T out of the garage for the first cruise of spring, and find they have no brakes the first time they hit the pedal.

To prevent that possibility and avoid that liability, is likely why builders went to adding residual valves where they did.

 

I partially agree with you IG, except as the pad wears, the caliper never fully retracts until you change the worn pads and compress the piston. Otherwise it just retracts enough to stop the pad from dragging on the rotor. Much different on a drum brake system. The springs in the brake system will compress the pistons in the wheel cylinder. Hence, the need for self-adjusters to keep the pedal from having to travel too far to apply the brakes. Did you ever replace brake pads on your car and the first time you press the pedal, it travels quite a bit. Pumping it a few times, you’re good to go again.

 

Check the fluid on a regular basis, that is the clue..............I'm just saying, keep it simple St***D..........

 

In everyday use, they'd never fully retract, fluid drain-back is a slow process based mostly on the friction between the piston and the bore, but if the vehicle is parked up for an extended period as the fluid drained back into the MC, the piston would retract either until the MC was full, or it fully retracted

 

I would agree with that on a drum brake system. I’m not so sure on a disc brake system. What is there in the disc brake system that would push the piston in the caliper back?

 

If the fluid drains back to the MC because of gravity, creating negative pressure in the bore, the piston has to retract as atmospheric pressure wants to push it in, admittedly an extremely slow process, and impacted by piston to bore friction, but that's the mechanism

 

In my (way too) many years of driving, tinkering or fixing cars, I’ve never experienced a soft or low pedal from a car sitting too long unless there’s some kind of failure of a part of the brake system. My bucket sits all winter and in spring the pedal is as good as it ever was.
We all have opinions. They’re like belly buttons. We all have one.
At this point I’m done. I don’t want to get into a pissing match.