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1983 R100 Brembos . Pick some brains here.

I remember a friend had a similar problem with his ATE's in the late 70's, but have forgotten what his solution was.. Currently my friend says the front brake has slowly slowed him down until it was impossible to even push the bike.
Cracking a bleeder nipple releases the pressure build up and he can continue. This doesn't happen every ride, only three or four times over the last summer.

I am assuming the problem is in the master cylinder rather than the caliper? I had helped him with a master cylinder rebuild kit in the spring. Possible for something to have blocked a hole somewhere, but only intermittently?

The return hole in the master cylinder bore might be clogged. Pluck a piece of wire from your wire brush and poke it in and out of the hole to clear away debris. If this doesn't solve the problem, Its possible that the brake hose(s) are delaminating inside and a flap of rubber is blocking fluid return. Hope this helps.

Clarification: The fluid return hole should be accessible at the bottom of the fluid reservoir.

+1 on the blocked return hole, same thing happened on a friends GS. His had so much crud in it we actually couldn't see the second hole in the bottom of the fluid reservoir. We used a turkey baster to suck all the fluid and debris out. Note: one thing you don't want to do is pump the lever while you are poking the hole, there is a seal in there which is sliding across it as you do. I used a bamboo kabob skewer, stronger than a toothpick, so less likely to break off, and less harsh than the wire wookie suggested, so less likely to raise a burr on the inside edge of the hole, causing the seal to leak.

Sometimes old rubber hoses fail internally and create blockages that can be intermittent, creating little flaps in the hose that can cause strange behavior. Most rubbers lines are supposed to be changed every four years OEM spec.

Thanks guys,
Another local friend has also responded to me, wondering whether it could be old hoses. This had been the problem with his RS.
Will get my friend to look at the return hole and also consider replacing the hoses.

Another vote for the return hole. It is a very small hole.