Be sure the piston is PERFECTLY straight into the bore. Give the lever a small squeeze to make it straighten up if you think it's crooked. Should only require medium effort with the C-clamp, don't force it.
It could have a rust/scum ring built up inside too. If the fluid is coffee colored, it may be worth your time to pop the pistons out and inspect/clean/de-rust. The coffee color is a result of water in the fluid. Very few people change and flush the fluid in their routine maintenance.