I did some research on brake fluid recirculators recently.
Google came up with a bunch of people selling them. After a while it became apparent that there were only three places that actually made them - DPI (Dan Press Industries), Howard Stewart, and Brembo.
I made a call to DPI first. I asked if they had ever used one of their recirculators with ABS. They said nobody had ever done that as far as they knew. I asked how theirs differed from the Howard Stewart and Brembo units. They said they'd never seen a Brembo, but they'd bought out Howard Stewart's entire line of braking products, and they had ceased production of the Stewart recirculator.
I questioned how the recirculator worked. Basically, their "Sure Stop" device mounts up by the master cylinder. It splices into the front brake line near the cylinder, before it splits to each wheel. New hard and flex lines run down to the bleeder ports on the front calipers.
The doohickey is basically a pair of check valves. When you push down on the brake pedal, the master cylinder forces fluid *through* the caliper, all the way back up to the check valve, which then closes, and pressure builds in the caliper. When you release the pedal, fluid can flow back from the extra lines into the caliper, and the cooled fluid cools the mass of fluid in the caliper. They claimed the pedal travel was not noticeably unchanced.
The DPI guy also said the system was "self-bleeding." If the lines are properly run, I'd believe that. He said they hadn't sold any in quite a while since the stock guy guys didn't use them any more.
I called another brake vendor and talked with the tech there. Their web site said they were a Brembo vendor and they actually had a picture of the Brembo product on their site, but their tech said they'd never actually had one in stock. They listed both the Howard Stewart and DPI recirculators on their page, but they claimed "nobody uses them any more - they never did work" and that the pedal travel went up quite a bit due to the extra flex lines needed. He didn't know of anyone who'd used them on an ABS car, and was firmly of the opinion that the recirculators were just a gimmick.
I sat down and made some guesses about the amount of fluid displaced by the caliper during a normal application. I'm coming up with something like .2cc, depending on retraction and pad knockback. That's about the volume of a pencil eraser, and that fluid is coming from the line right there at by the caliper, so it's fairly hot to start with.
Best as I can figure, the tech guys were probably giving me the straight scoop - they probably didn't do a damned thing other than make the pedal travel longer.
Now for a curveball - DPI sells a "Sure Stop II" recirculator. It's a check valve setup like the SS-I, but it incorporates a spring-loaded piston connected to the *back* brake line. It takes fluid from *both* master cylinders and sends it all to the front until the line pressure overcomes the spring, and then the shuttle valve directs the fluid to the rear brakes. The purpose is to reduce the pedal travel with the extra fluid. This is, interestingly, similar to the "quick take-up" master cylinders GM used in the 1980s, which used a stepped piston and a check valve to reduce pedal travel. The SS-II might be useful on a heavy car with mechanical brakes, to let you use a higher pedal ratio without an absurdly long strong.
So, in the end, the recirculator thing turned out to be a dead end. You could probably get a bunch more use out of a piece of vacuum cleaner hose wired to the caliper as a cooling duct.
Now, if you were to run the recirculator line back to the master
cylinder instead of splicing it into the pressure line, you could add a
tiny low-pressure pump and circulate a little fluid through. But even 2
PSI can cause the pads to drag, according to Puhn's "Brake Handbook."
Brake fluid is pretty viscous, and 10 PSI with my pressure bleeder sure
doesn't move much fluid. But it'd be a hell of a lot more fluid than a
"recirculator" moves...
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