I was looking through the recent post on residual valves and the trouble they create during the install or the little fact that one manufacturer is putting out a faulty device and to my knowledge hasn't done anything about it to date. At least it doesn't seem like they're doing anything because the complaints keep showing up on hot rod boards and here on the HAMB. As y'all know, you need a residual valve when the M/C (Master Cylinder) is below the level of the front calipers. Or do you? So I was thinking . . . dangerous I know, but instead of a residual valve that may or may not work I wonder if gravity would do it? It's apparent that not much pressure is involved in the siphoning process when a residual valve is not used. Considering that a 2# valve will hold the pressure back and the little fact that there's usually a 'fudge' factor built in to components I'd be surprised if there was a 1/4# pressure involved and it's probably less than that. If we came off the M/C with a brake line and ran it up as high as it would go on the firewall then turned back down to the frame and set the line up so the wheel cylinder could plug into it as usual the 'head' gained from the height of the brake fluid may be enough to keep the siphon from starting. Granted, we'd need a T fitting at the top so we could purge air out of the system at that point, but that wouldn't be a big deal. If 3/16" (OD) tubing didn't have enough pressure 1/4" could be used for a pressure increase. And we may not have to go all that high. Hanging pedals are most times, not too high up on the firewall and they're not subject to siphoning fluid from the wheel cylinder. I feel like I've thought this through carefully, but if I've missed something I'd be glad to hear comments. In any event I think it's worth a try on the 31 roadster project and there's lots of places to hide a tall vertical pair of lines....
sounds like i need to try that.. I have the residual valves sitting in my tool box. Havent wanted to put them on yet.. just keep pumping the brakes.. ha ha
Take about 10 feet of 1/4" ID Tygon tubing and siphon some gas out of your Deuce to feed your cactus mower. While it's siphoning, lift the middle of the hose as high as you can reach while keeping the outlet lower than the gas level in the source tank. It's still siphoning, right?
Part of the reason for the Residual Pressure Valves has nothing to do with heights or gravity. Brake rotors have a tendency to knock the pads, and therefore pistons back into the calipers. This is caused by a number of phenomenons, including but not limited to slightly warped rotors, flexing caliper mounts, loads through the wheel bearings during cornering that cause the rotor to move slightly, etc. When any number of these things occur the pads can push the pistons back and that creates a "long" pedal. The 2lb RPV helps to hold the pads out against the rotor to help eliminate the excessive pedal travel. With drum brakes the 10lb is used to overcome the pressure created by the brake shoe return springs, again keeping pedal travel to a minimum. Do all cars need RPV's? Not necessarily. Some OE masters have RPV's built in, especially many early drum brake dual masters. Some applications seem to work without due to master cyl bore size versus caliper size. Some times you just end up with a pedal ratio that seems to over come the extra travel created by not having an RPV. NASCAR teams will pry the pads away from the rotors during qualifying on Super Speedway's to reduce drag and tell the driver to not touch the brake pedal unitl after the qualifying lap. Then they radio them to pump the pedal because of the extra travel.
I'm amazed you know I have a cactus/tumbleweed mower. As noted, I thought the pressures involved would be so low that a siphon couldn't start. Good point as always Dr J.
Excellent points and what I was looking for in the way of additional information. I understand about the drum brakes and somewhere along the line read that it takes about 65-75# to overcome the brake springs. The guys with firewall mounted M/Cs seem to do ok, but from what you've written it appears that they need a residual valve as well.
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