Rear Master Cylinder

[gaj1917]

At the end of the driving season last year I thought the rear brake pedal felt a bit spongy on my '84, but still had lots of stopping power so didn't think much of it. Recently, after replacing the rear tire, I checked the rear brake to make sure it was okay - nothing, rear or left front! I bleed and re-bleed. No bubbles. Still nothing, but if I frantically pump the rear brake pedal I can get some brake action. I guess a faulty rear master cylinder is a possibility, but I rebuilt the innards of that last year. I recall @cowpuc recounting fixing a problem with the master cylinder while on the ride, but I can't find the thread now. I have SS brake lines BTW.

[Yammer Dan]

Trapped air can give ya fits!!

[bongobobny]

Possible clogged proportioning valve...

[gaj1917]

Possible clogged proportioning valve...

 

Thanks. Well buried up front of course.

[bongobobny]

No, the proportional valve is connected to the rear master cylinder and is just a spring loaded valve that delays the fluid going to the front brake by a few milliseconds to prevent the front brake from locking up before the rear brake in case of slamming on the rear pedal. There is also an inline orifice valve to the front brakes near the steering neck that limits the volume of hydraulic fluid to the front brake only...

[gaj1917]

No, the proportional valve is connected to the rear master cylinder and is just a spring loaded valve that delays the fluid going to the front brake by a few milliseconds to prevent the front brake from locking up before the rear brake in case of slamming on the rear pedal. There is also an inline orifice valve to the front brakes near the steering neck that limits the volume of hydraulic fluid to the front brake only...

Thanks for the clarification. I was confusing the proportioning valve with the orifice valve. Much easier chore to haul the master out, so will start there.

[bongobobny]

Yah! That other valve, I forgot what it is actually called, only effects the front. It is just a simple orifice and rarely causes problems, but I suppose it could get clogged too...

[gaj1917]

I wasn't looking forward to removing and opening up the rear master cylinder, so decided to let the bike set for a week and then try bleeding the brake lines again. This time I got serious and used my MityVac to suck out brake fluid. I managed to get a good gulp of air at the left front anti-dive unit, and just a bit at the front and rear calipers. I don't know where that air was hiding, but I have brakes now.