Brake LIne Pressure Question

[Argess]

No offense intended Bob. I merely wanted Patrick to know why I chose DOT 5.

Didn't have much time this morning, but I may have made some headway. Reading up on DOT 5 it turns out many people have had trouble bleeding their brakes. One guy "tickled the pedal" and re-bled every day for two weeks before he was satisfied with the pedal. Another took 2 months playing with his on and off.

A lot of speculation has been presented stating that the fluid is very tenacious keeping air bubbles attached to it.

So I wondered where this air mught be. Suspension most likely, but then I thought of the space around the caliper piston. Even at 0.005" gap, a 2,15/16" piston could have enough air around it (incredibly unlikely) to take up 3/4" of MC travel.

Now all vacuum bleeding does is pull fluid from the reservoir to the caliper bleeder. I wanted more of a bell jar test. So I put a piece of wood between the seat and the pedal... just enough for the piston seal to pass the compensation/fill hole thereby isolating the front brakes from the reservoir. Loosened the bleeder and applied vacuum. Yes siree... air started coming out!

Next step is to take the caliper off and lay it out flat with the piston bottom facing skyward and repeat the procedure. Then install it and pull any air that crept out of the piston/bore clearance.