680 k case air problems

I have a 680k backhoe. can someone give me some trouble shootting tips on the air system. Last winter there was not enough air to release the parking brake. I thought somethin got frozen so wait things to warm in the spring. I did wait for warm weather and things started working again but air pressure build-up took a little longer. Today there is not enough air to release the parking brake like last winter. I needed to move the backhoe so I connected annther airline to the system to just relese the parking brake. When I did that the air being pumped in went throgh the backhoes compressor and came out the air filter that supplies air to that air pump.

Does the air compressor have a checkvalve built in it?

If it duesnt, where is the checkvalve?

If anyone has any ideas, I will appreciate a response to this thread.