master cylinders

The front port is there to ensure pressure does not build up in the circuit when the brakes are at 'rest'. If this port gets plugged, then when you release the brakes any heat in the system can cause the fluid to expand and having nowhere for the increased pressure to be released, would move the slave pistons out engaging the brakes - leading to more heat - leading to more pressure , eventually causing the brakes to lock up.

The rear, larger port is how replacement fluid gets into the circuit as the pads wear and the slave pistons move further out of their bores (of course, 'replacement' fluid can also get in through the smaller port), The fluid moves from behind and around the piston cup into the circuit and having a larger port allows the replacement fluid to be drawn from the reservoir more freely ensuring any pressure reduction in the high pressure side of the circuit (after releasing the brakes) doesn't prevent the brakes from fully releasing quickly.

Got it, thanks

Gents I am trying to make 1 decent rear master cylinder out of two incomplete ones I have and cannot see if the hole that allows fluid from the reservoir is clear or not I have it disassembled at the moment so I suppose if I put some fluid in it with no piston in it should come out if the hole is not blocked, also is there one or two holes in the rear master cylinder, apologies if these are stupid questions but I have not had a lot of experience with master cylinders but seeing as I am locked in seems a good a time as any to learn
many thanks

Hi, the 2 holes mentioned in this thread are in the front master cylinder. I don't think you can see any holes in the rear, at least not in mine.

There are 2 holes but not obvious because they are hidden behind a black plastic plug. The holes could be cleaned or checked from the bore of the piston with a 90 degree piece of wire and flushing with probably lacquer thinner. See pics


This pic shows the location of the plug. They had to have a place to machine the 2 holes in to the piston bore then plug.

This shows the inside of that cavity with the plug removed.
I pulled these pics from a thread back on June 17 2019 thanks to Lucien Harpress for the post. He learned the hard way what happens to plastic plugs that you don't know about when you refinish your rear master cylinder with caliper paint and bake cure @ 200 F. Ooops

Thanks, I'll have to check my rear cylinder now. Yes I saw the picture of the melted plastic plug.