Mark III rear brake sticking on

This is a bike I am working on as a favor, 1975 Commando MarkIII. It had been sitting for 20+ years when I got it, funny thing is it was parked just a few miles down the road from where I live...funnier yet it is only 99 serial numbers away from my own Mark III.

Here is my problem. After getting it running I took it for a test ride and found the rear brake sticking badly. I rebuilt the caliper and still have the same problem. I rebuilt the rear master cylinder and still have the same problem. As soon as I crack the bleeder on the rear caliper the brake frees up. I took very careful measurements when I disassembled the rear master cylinder body. I have read that if the piston assembly is threaded too far into the body it could cause these symptoms, however I am confident my measurement is correct.

What else could be causing this?

If you are absolutely sure the master cylinder has been set correctly, so the pressure is venting to the reservoir......

http://www.classicbike.biz/Triumph/Main ... Norton.pdf


.......then it has been known for old rubber hoses to collapse or delaminate internally, the loose section can then act as a 'one-way' valve.
All original rubber brake hoses should have been replaced by now.

L.A.B. said:

If you are absolutely sure the master cylinder has been set correctly, so the pressure is venting to the reservoir......

http://www.classicbike.biz/Triumph/Main ... Norton.pdf


.......then it has been known for old rubber hoses to collapse or delaminate internally, the loose section can then act as a 'one-way' valve.
All original rubber brake hoses should have been replaced by now.

Click to expand...

You are correct sir! I went back to revise my work after taking a little break (County Line IPA!) I remembered finding the rubber brake line plugged solid on my Mark III when I was resurrecting it after I bought it.
Funny thing was on this one I was getting fluid out of it when pumping the brake pedal. I separated the hose at the bracket where it joins the solid brake line at the caliper and tried blowing compressed air through from the end of the line at the M/C. I was getting air but not much. I pulled the rubber hose and tried again...Definitely blocked. It must have been passing fluid through but was so blocked it wasn't letting enough return. I wonder if I could get an auto parts store to make one?

dirtymartini said:

I wonder if I could get an auto parts store to make one?

Click to expand...

Yes, I expect so, as the fittings are standard 3/8" x 24 UNF, 45-degree (SAE) flare.

You may need to adjust the rod pressing on the piston of the master cylinder. If it's too far into the cylinder it will cause the piston to mask the passage to allow the fluid in front of the piston to return, or expand, back into the reservoir.

I had the same problem and was sure I had reassembled the master cylinder as measured. I was about 3/4 of a turn in too far, blocking the return port.

Also noted that afterward my rear brake would not activate the brake light. The extreme pressure built up in the system ruined the pressure switch.

Ride On
Dave

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