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Rosenhan brakes and silicone fluid?

asport22

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Anybody have any experience with the old Rosenhan drum brakes (Forerunner to Matco, no longer supported by them)? They seem dead-nuts simple, but I'm not having any success making them work on the Jodel.

I honed the wheel cylinders and replaced the rubber cups. That seems good - no noticeable leaks, pedal doesn't leak down. I replaced the master cylinders with Matco, because I couldn't get all the pits honed out of one of the original Rosenhans. I'm running DOT 5 silicone fluid because the Matco masters have aircraft o-rings and the wheel cylinders are automotive cups (never can keep straight which is neoprene and which is butyl). Shoes are adjusted to just clear the drums with no pressure.

The pedals are firm, but there is just essentially no friction. The ramp at our airport has a VERY slight slope, and today, I could not stop on that slope with the engine at idle.

I've gone through about four iterations of brake pedal geometry, and am pretty sure the current one is OK - not perfect, but has solved all the issues of the previous ones. However, with the third iteration, one of the brakes worked well - except for the part where it didn't release when I took my foot off the pedal. Turns out the Matco master cylinders have to go all the way to the top to relieve pressure and my linkage didn't always allow that.

Does anyone know what sort of pressure drum brakes need? I think I remember that drum brakes operate at quite a bit lower pressures than disc brakes, and since there was that brief moment when one brake worked well, I think I am generating adequate pressure. I am considering ordering the Matco intinsifiers, but it's a lot of work to pull the master cylinders to install them, so that's a last resort. (And I don't know if it will help.) (Well, the last resort is changing the wheels/brakes out completely to new disc ones, but I'm reluctant to do that too.)

"Silicone" is kind of a broad term, but I know it is often applied to stuff that is very slippery. During the bleeding process, the silicone fluid gets all over everything in the drum/shoe area. Could it be lubricating the drums/shoes enough to reduce the friction to nearly zero? At various times, I've cleaned the shoes and drums with Brakleen and also sandpaper - but maybe not after the last round of bleeding.

Thanks!

Warren
 

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