GT550 Master Cylinder

[markc522]

I have a '74 and had a leak at the master cylinder, the only piston/cup set I could find was on ebay and i installed it. Now, the master builds pressure and will not release the caliper. I started by cracking the bleed screw and it released, and it worked, then I pressured it up again and cracked the banjo bolt at the master and it released again. This tells me I dont have a problem with the caliper or the hose, its with the new piston kit. Found a site that had the dimensions (14mm dia, and 51 something long.) I made the mistake of tossing the old kit, but the new kit was 'close' but not identical.

Anyway, on to my problem. As I bleed, the air comes out, everything works fine, but toward the end the lever keeps getting more and more solid. With the front wheel off the ground, the free spin just keeps getting less and less until the front wheel is locked. does anyone have any ideas before I just buy a GS master or know of the correct piston kit?

[karl pa]

In the master cylinder reservoir under the metal plate,there are 2 holes, 1 allows fluid into the piston, and another very very small hole that releases pressure from line. If the very small hole is clogged your front brake will lock.

[markc522]

Its clear. I did my best to drain the fluid and now have started bleeding all over again with a new unopened bottle of fluid and I have the exact opposite problem, now I cant build pressure. I cant even use a vacuum on the bleed screw and get anything fluid to flow. Im thinking a blocked line? About to order a new universal line and see if that gets me on the road.

[sportston]

Once had that on a customer’s car years ago, caused by the rubber brake hose disintegrating internally.

[Harrymail]

I found the pattern master cylinder kit useless. Had the same problems. Fortunately I cleaned up the piston that was 50 years old and just used the seals and clip from the kit. Bleeding was then straight forward. I think the new piston was jamming in the cylinder as there were witness marks suggesting this. It did move smoothly either. I think the pattern ones are a poor copy.

Paul

[Alan H]

Unless looking original is a priority, there are lots of alternatives - GS (many models) are very similar, Bandit models, even Honda m/c's plus several chinese master cylinders work reliably at very good prices. I have a pair on the 550A (Earwig) for the twin front discs & hydraulic clutch setup. Worked fine for the last 3 years.
https://www.kettleclinic.co.uk/kcforum/ ... 90#p100690