[TR] TR4 Master cylinder problems

[Angelo Graham]

Hello List:
Working on a pair of TR4 master cylinders (brake & clutch) and trying to 
fit rebuild kits. Have the pistons free and the circlips out, but I 
can't seem to extract the pistons. They appear to be hitting a something 
at the circlip end of the bore. I can't feet any wear ridge or anything 
that might be stopping the pistons. Am I missing a step here or is it 
the case that it doesn't take much to prevent the pistons from exiting 
the bores. Have tried much soaking, etc. and trying to lightly 
emery-cloth the part of the bore that I can reach.
Any tips or is this a common problem??
Thanks in advance,
Angelo Graham