Moentrol 3510 m pact only warm

[Drake4646]

my new moentrol 3510 m pact valve was installed by plumber but shower was not used in 2 years. Finally finished shower and only hot water is coming out at constant temperature butt varying pressure as it spins. Help! Attached is a picture.

 

[Drake4646]

my new moentrol 3510 m pact valve was installed by plumber but shower was not used in 2 years. Finally finished shower and only hot water is coming out at constant temperature butt varying pressure as it spins. Help! Attached is a picture.

Ended up being the balancing valve. Used a flat head screw driver. Luckily it came out. Then I took the back cap off of the balancing spool. Got it off using a small screw driver. The black piston was stuck. I got it unstuck by lightly hitting it with a screw driver. Sanded the inside. Put plumber grease back inside. The pistol then moved freely. Put it back together and in valve. Good to go.