I had a similar problem a couple years ago. My first stop of the day needed me to pump the rear up, then it was fine after that. There was a leak somewhere. I pulled the whole system apart, put new seals in the MC and caliper. Put it back together and I couldn't get it to bleed out all the air. I went through a whole (small) DOT5 bottle. Then I found a very small leak at the coupling between the rubber and hard line on the swingarm. I got a new crush washer at the auto parts store and it bled in about 2 ounces.

I'm a firm believer in DOT5. The insides of the MC and caliper looked new, despite having 20-something year old fluid in them. I last checked the fluid level in the rear caliper reservoir sometime in the 80's or early 90's. Note that DOT5 is not the same as the DOT5.1 or DOT5.2, whatever they use now. The DOT5.x is actually about the same as DOT3/4. You can get DOT5 by the gallon for about 30 bucks on ebay. It's the same stuff the military uses in M1 Abrams tanks.

 

Thanks Graham, is the part that the pedal arm attaches to a problem area? The rectangle peice held on with 2 nuts and bolts and the brake switch is screwed into.

 

Thanks Doc , its funney how the rear brake always tends to get the lack of attention.

 

Dont forget to do some PM on the brake pedal pivot and linkage too. Mine was dry as a desert, took it apart and cleaned out all the old petrified grease and re-greased after assembly. Its amazing how much better feel the rear brake has now.

 

Lynn, that rectangular piece is the master cylinder! Believe it or not, when I replaced my entire brake system some years ago, that was more expensive than the handlebar one. Also connected to it is a pipe from the reservoir, which is behind the left side cover.

The pedal arm can dry out, as Mountainkowboy commented on. So well worth removing it to clean and lube. My bushings are worn and I reckon on stripping mine again to see if I can improve things in there.

By the time you've been through that lot you won't recognise your bike or brake!

 

Ok guys I'll have to dive on.
Funney you mention bushings, last year I changed the bushings on the inner primary cover for the shifter and thought that would do the trick as the shifter was super loose all the way back to the tranny linkup, didn't do the job, over the winter I changed the shaft and its absolutely amazing how snug the whole assembly went, the old girl was happy.

 

I fabbed a new bushing for the shifter at the inner primary. I had an new OEM one (two, actually), but it's just a little net like thing. I made one from Delrin on my lathe that is th length of the hole through the inner primary plus the slack on the end. Super smooth, no more rattling and bouncing.

 

Ya still have a bit of a gap on the end.

 

Yea, the factory tolerances were a little wide!

 

Can someone tell me which direction the open rubber cup faces in the rear master cylinder? The cup has an open end and a flat end, I'm pretty sure it was flat end out to the
Piston.