'83 Aspy rear master lockup

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kerryb

kerryb

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The brakes on my '83 Aspencade with the linked brake system would slowly fill with pressure until the caliper and pedal locked up. This happened slowly as the brakes were used and the bike heated up in normal riding. The only way to get back on the road was to crack open the bleeder on the rear caliper or the hard line at the master cylinder. After rebuilding the rear master cylinder 3 times, the third time with GENUINE Honda piston and seals, the relief port in the master still wasn't uncovered by the seal in the relaxed position. This meant that pressure buildup from heat expansion couldn't be relieved back to the reservoir. I'm sure I got the right parts from the dealer, I'm sure the relief port is open, I'm sure the lines are bled well enough, I'm sure the pressure wouldn't relieve because I pressurized it on the bench with the reservoir hose removed!
I finally fixed the problem by putting the M/C piston in the lathe and taking 1/16" off each end to make it SHORTER. Now the master cylinder works correctly and hopefully when I put enough miles on it to prove myself right , I'll be pleased, but still puzzled why no one mentioned this before.

The brakes on my '83 standard w/o the linked feature give me no problems at all.
 

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I removed, drained, inspected, and flow-checked all the lines involved in the rear portion of the brake system. I have not yet rebuilt any calipers or flushed/bled the lines to the front caliper. Since the bike wasn't on the lift when the problem occurred, I could not be sure the front brake was free or locked up. I assume the front was locked also because the pedal was rock hard and wouldn't budge. loosening the M/C nut or the bleeder at the rear gave temporary relief. Back on the bench, I measured the distance from the circlip to the relief port and compared this to the length of the piston and seals to determine that the relief port was still in the main pumping area of the piston travel, thus no relief was possible.

As was said already...Hmmm,...very curious indeed. Could this be part of the brake recall that I have seen mentioned but never explained?
 
I have to say that this is the first time I have heard of this. Just wondering if what you have really is a stock 83 MC or could it possibly be that you got something other than an 83 MC that a PO put on the bike in the past and then the 83 parts are not playing well with the others?
 
That was part of my thoughts also.....our he got the right part fir the year and model.. but the master wasn't the correct one... maybe a donor bike part? ..
 
Agreed...The M/C must be from something else, which considering the condition of the bike when I got it is very odd. So the next step is to determine what M/C I do have. Are the markings on the outside enough to find this out? If so, where do I look.

I don't really like the idea of reduced wear surface on the piston. It could lead to premature wear on the cylinder bore, and that would be another cluster to deal with.

"just for fun" kb
 
:head bang: :rant: that is the same problem i'm having. i've taken the master brake off 2 times now and it still freezes up after a couple pumps of the foot brake. one question how and what do you use to clean that return line? :thanks:
 
I guess I missed this post earlier - Kerryb, the master cylinder in your pic is an '83. '82 and before used a banjo fitting and flexible hose, and '84 and later have 2 outlets with an internal proportioning valve.

Make sure the brake pedal is not binding at all. If it sticks, the master cylinder piston won't return fully and expose the return port, causing pressure buildup. Also make sure there's no corrosion on the piston rod or steel disk. These could also keep the piston from fully returning.

That return port is tiny, and it takes patience to get it clean. A bottle brush in the bore, a thin, stiff piece of wire, lots of cleaning fluid, compressed air...whatever you can find. Shine a light into the bore and you should see a large and a tiny hole looking into the entry port.
 
Just another tip to add:

Be sure the rear brake pedal is not hanging at all! It has to move smoothly and be able to come all the way up on it's on. If it doesn't move smoothly and travel all the way up, it will pressurize the brakes while riding causing the rear to lock up and can only be released by loosening the banjo bolt connection.

:oops: oops...(Steve covered this already)
 
well now it seems that I have a standard mc or maybe an 82, cause after taking everything apart again and getting the relief hole cleaned out, the brakes are still locking up. time for some lathe work.
 
nope I have an 83. what I did to allieviate the problem was to drill the return hole out to .020, it was plugged real good. I then took caliper apart and checked each piston to see if they traveled by themselves. Found that one piston was really tight in the caliper, the p.o. took a hammer to the side of the caliper and actually deformed the bore causing the piston to bind. After some 400 grit sanding, the piston is free and the brakes work great now. :thank_you:
 

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