if this is the case,, then once you have brake failure,, you can't pump the brakes to inject more fluid... as it returns to the reservoir everytime you lift off the brakes....
in fact,, if I am correct.. if the pads travel a given amount..with every pedal stroke.. once that amount is exceeded by wear in the pads and rotors.. it is possible to NOT have enough fluid to move the pads enough to contact the rotor any more.. even if the pads and rotors look new..
that means the master needs to move enough fluid every time to fulfill the needs of the calipers when worn out.... so a new system would move the pads 1mm,,, but as it wears .. their could be 3mm play on each side of the caliper.. or 6mm total more to move..
Actually, that is one of the reasons you have a residual pressure valve in some systems. On disks its usually a 2psi valve IIRC, and drums its 10psi IIRC to help overcome the return springs.
In disks, what happens, is that as the pads wear and the piston has to move out farther, the residual pressure valve keeps that 2psi in the line as the pedal returns. In most systems the 2psi is used to keep the pads in light contact with the rotor and take up slack from wear.
Your master cylinder can pump more fluid into the caliper every time you press on the pedal then is needed to activate the pads against the rotor and the pads wear a miniscule amount every time you brake. All this is very dynamic and always changing depending on wear, heat, how hard you hit the brake pedal, etc. One thing that is pretty static is how much pressure in the line is required to keep the pads lightly against the rotor - about 2psi.
So you hit the brakes - fluid is forced by the movement of your leg against the master cylinder piston, goes through the residual pressure valve, through the line, into the caliper, which makes the piston move out, forcing the pad against the rotor. Due to hydralic multiplication, you can end up with 1500psi at the caliper in some situations, but for a normal stop, its closer to a few hundred psi. This is a simplified version of how your brakes work.
During the stop, the pads wore a little bit, as did the rotor, and now the caliper piston had to move a little bit farther out of its bore to engage the brakes.
Now you release the brakes, and the master cylinder piston returns to its resting position - in the process the fluid is sucked back into it through the lines, through the residual pressure valve - which closes at 2psi - trapping a little extra fluid in the calipers to keep them in light contact with the rotors.
This is how most systems work but there is always exceptions. There are low drag calipers which fully retract the pistons every time so that the pads don't drag on the rotor. There are also those which use the pads for the e-brake, which requires a different system since it is mechanically activated, not hydralics. These all use different systems to compensate for wear.
Oh yeah - when I went from 4 wheel drums (adjusted for zero drag) to 4 wheel disks on my old 68 dart drag car - no change in ET.
-- Edited by RacerRick on Sunday 5th of July 2015 10:27:33 PM
I guess the other problem here...... most master cyl are set up so the pedal has a slight gap between the pedal push rod, and the piston..
this application is on a hand brake car... no foot pedal brakes... so, I don't know where to set the handle so there is that gap quite yet... and I am thinking the 2 psi valve is an important thing ,, so it does like you said...."master cylinder can pump more fluid into the caliper every time you press on the pedal" instead of drawing it back into the master cyl..
I'll check out a few cars in Cayuga this weekend.. see if they are running check valves.. as there are quite a number of little things in the adjustment details to get this set up to work.
Usually the residual pressure valve is located in the master cylinder itself. GM's are this way, and I think mopars are also. I don't know about fords.
The pushrod to the master cylinder or booster should always be adjusted to zero gap, then you can adjust it to set your pedal height.
The weight of the pedal will keep the rod in light contact to the booster or master cylinder. Some boosters have a clevis to connect to the pedal, ensuring no gap. The pushrod from the booster to the master should be adjusted so it is just short of the master cylinders piston so it won't cause the brakes to drag.
I think everything Racer Rick said is right but I did read on the Master Power Brakes site that you could get away without the 2 psi front residual valve as long as your master cylinder was mounted above the height of front calipers.
I didn't use one on my car and the brakes feel just fine. Even without the residual valve, the front pads still drag on the rotor a bit. I'm guessing that the 2 psi valve wouldn't add very much drag though.
I did use the 10 psi valve for my rear drum brakes. I think the 10 psi is just enough to overcome the return spring of the drum brakes.
