Problem with brake system of dodge t110

[gordon]

It seems very unusual to me to oil that cylinder with the engine oil, but if that is what the manual says ...

The return spring will be to pull the PEDAL back, not the master cylinder pushrod.

If the return spring is in place, pulling the pedal back, then it comes down to two possibilities;

1. You are not getting full vacuum on the side of the piston when you release the pedal - though you say you have already checked this.

2. The cylinder piston is sticking in the bore.

I would suggest you clean and oil it again, then run it again and see if it sticks again. As long as it is easy to clean I'd do that a few times before I thought of trying anything else.