brake questions: diaphragm, rear, rain

1) what does the rubber diaphragm thingy do in the master cylinder? other than help seal it shut, does it keep fluid from sloshing in such a strange shaped reservoir??


2) what's up with my 97 Jimmy's rear drums brakes? I've bled them twice and barely get any fluid. It just seeps out of the bleeder. It's like the front brakes are doing 90% of the work. I've got a full M/C and no visible leaks. When I crawl under the rear of the truck, I can hear things working (mechanically) when I have an assistant pump the brakes.


3) why do my brakes seem more sensitive for a little while after it rains? rain washing the rotors ?

3 is simple, rust... you get a thin layer of rust during rainfall on the roter, and you burn it off, once burned off the breaks become less touchy...
number 2 is a little more difficult, could be lots of reasons master, prop/meatering valve, restriction...
as to number one... why do you have your master apart?

3 is simple, rust... you get a thin layer of rust during rainfall on the roter, and you burn it off, once burned off the breaks become less touchy...
number 2 is a little more difficult, could be lots of reasons master, prop/meatering valve, restriction...
as to number one... why do you have your master apart?


my M/C isn't apart ... I just took off the cap to refill and top-off while bleeding. There is a rubber diaphragm thing that covers the whole top of the M/C ... not just an o-ring or seal ...


I was thinkin' the opposite on the rotors ... I was thinkin' the rain washed away the brake-dust, thereby makeing the rotors clean and more grippy and sensitive ...

... thoughts ... ???

That rubber thingy is normal. I think that it's more or less a seal.

Your front brakes do 75 - 90% of the braking. Although it doesn't explain why you have so little fluid coming out of the back bleeders. Have you tried adjusting the rear brakes? I'm not saying that this would help bleed them, but it sure won't hurt

break fluid is knowen as hydroscopic, this means it absorbs water right out of the air, that, as far as i know is an extra seal to keep moisture out... and no, the break fluid isn't washen away, a layer of rust is building up and this grabbs much better then the intended use of them... after the non-directional finish wears off, they are just glazed, the break dust doesn't effect the breaking in the mannor your thinking of

60-40 front-back, respectively, and even though the backs to more work, as obp says, there should still be volume of fluid commen out the back bleeders, unless maby a popped break drum

THe diaphram seal gets sucked down into the reservior as the brakes wear so the fluid does not move and create bubbles or aerate - keeps a closed system. As for those rear wheel cylinders they are at the ends of the lines and I would bet they are full of crap so change them out - cheap part. Lube or change all that rear hardware too if not already done and don't forget to lube behind where the pads rub on the plate area.