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Slow pressure buildup on front brakes
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There was a bit of a thread on this on the mail list. My experience was that my front brakes would slowly build pressure (over one to three days) even though I had avoided using them, except when absolutely necessary. I had cleaned the calipers, changed the pads, bled the system well and (I thought) cleaned the master cylinder well.
The problem lies in the master cylinder. With the top off the master cylinder (as well as the rubber diaphragm inside) you can see two holes in the bottom of the master cylinder. You can see the holes best if you remove the plastic reservoir that is held in place by an O-ring. To remove the plastic reservoir just twist and pull and, if necessary, pry gently with a broad, flat screwdriver, being careful not to damage the plastic.
The holes must be clean. The larger of the two, located closest to the mirror, is relatively easy to clean and to see. The other hole, farthest from the mirror, is within a small, scallop-shaped depression in the surface of the master cylinder. The first time I cleaned the master cylinder I missed it and the problem persisted. The second time, I managed to open the hole by reaming it with a 0.008" wire (a piano wire). This started the hole and I completed the cleaning with a 0.014" piano wire that seemed to just fit perfectly. I re-washed the entire unit in solvent and reassembled. It primed and bled easier and the slow pressure buildup no longer occurred.
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There was a bit of a thread on this on the mail list. My experience was that my front brakes would slowly build pressure (over one to three days) even though I had avoided using them, except when absolutely necessary. I had cleaned the calipers, changed the pads, bled the system well and (I thought) cleaned the master cylinder well.
The problem lies in the master cylinder. With the top off the master cylinder (as well as the rubber diaphragm inside) you can see two holes in the bottom of the master cylinder. You can see the holes best if you remove the plastic reservoir that is held in place by an O-ring. To remove the plastic reservoir just twist and pull and, if necessary, pry gently with a broad, flat screwdriver, being careful not to damage the plastic.
The holes must be clean. The larger of the two, located closest to the mirror, is relatively easy to clean and to see. The other hole, farthest from the mirror, is within a small, scallop-shaped depression in the surface of the master cylinder. The first time I cleaned the master cylinder I missed it and the problem persisted. The second time, I managed to open the hole by reaming it with a 0.008" wire (a piano wire). This started the hole and I completed the cleaning with a 0.014" piano wire that seemed to just fit perfectly. I re-washed the entire unit in solvent and reassembled. It primed and bled easier and the slow pressure buildup no longer occurred.
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