Caliper piston hard to compress--problem?
 

I changed the front brake pads on my car this morning.

Three of the pistons compressed fairly easily using a $3, 3" C-clamp. The fourth was a real bear. I ended up using a 12" lever arm on the C-clamp in order to get enough torque to compress the piston.

I did have the bleeder screw open.

Any idea as to why the piston was so hard to compress? Does it indicate a problem?

yes.
it indicates there is corrosion or rust in the bore binding the piston... usually it means the caliper needs to be replaced, but some have successfully removed the piston, and cleaned it, then put a new seal kit in.

Agree with VS.

That caliper will not provide any force to the pad on the last piston that had to be forced back in place. This will result in much lower brake force on the side with this caliper. It will also bend the hell out of the rotor.

Replace the caliper before driving the vehicle.

Rust. 4 pot calipers or single pot calipers? confused.

I'm not sure what a pot is on a caliper.

There are two pistons, if that answers your question.

I'm currently having the exact same problem on my 1980 300D so I commiserate. Went to change the pads yesterday and had a miserable time with one cylinder, just like you.

Came on here this morning to have a look around and see if it's actually worth the trouble to attempt the seal kit rebuild. Stinks, because the caliper itself looks so clean and pretty. NO rust. I mean none. AND the 19 mm bolts turned with no pain OR swearing. It was a little spooky, actually... But, the car sat for two years before we got it...so many problems when they sit.

Question, though...I would have expected to see uneven wear on the pads. There wasn't any. It was the inner cylinder of the rear left that was extremely difficult to compress, but that was worn the same amount as the inner pad on the rear right, and there's been no noticable pull on braking. Does this mean it's still got some functionality? Or are the effects of one misbehaving cylinder totally negated by the 7 other properly functioning ones?

I'm going to fix or replace it, don't get me wrong. It's not something that can ever get better on its own, and it WILL get worse and seize and leak and create a whole avoidable mess. I'm not even considering pretending I didn't see it and driving on my merry way. I once had a brake line go when I was behind a cop at a red light. Since then, I'm fairly picky about brakes! I'm just curious as to why there was no visible wear difference.

Two pistons per side? Without a car in your sig or bio its hard to tell what we are talking about. But yes, generally a sign of rust.

When I bought my 1980 300TD the PO told me you could only drive it a few miles before it would over heat and die. Diesel die? Hardly. But yes, it did. Because the calipers were rusted and the rotors would expand due to dragging calipers. Eventually they would get so hot and so much resistance would build that the engine would die.

The dealership told her it would be a new injection pump and cost $1200.....