#61215

The large piston in the vacuum chamber is sticking, and I have also heard that the replacement rubbers can be too thick/hard.

Sorry to disagree, but the other advice (from the professionals in the UK) is not to polish the bore, but instead give it a fine honed finish to keep the grease in the grooves. Also, try to get a silicone based grease. Since I took this advice, I have not had a piston stick on either of my two different booster rebuilds.