Service brakes don't fully release pressure

Boxcar's suggestion is a good one. Floor mounted pedals corrode & stick causing the brakes to hang up. Very common. Also make sure the trolley brake lever is fully returned.

 

Try pulling up on the pedal. The hinge pin can stick and not allow the brakes to release.

 

Penetrating oil works well pull the rubber up spray some around the plunger, also on both sides of the mounting pin.Try not to get it on your carpets.I endorse PB Blast.

 

Check to see if the relay valve/quick release valve exhaust port is opening up fully. That's a fairly common cause of brake drag.

 

If WD40 helped it’ll only be temporary. To fix it proper the hinge pin needs removed and cleaned up.
To do that you need to knock out the small roll pin. (On all of mine it was on the left side) Remove the hinge pin. Clean all that up with emery cloth or equivalent (sand paper). Clean the pin and bores I would roll up the emery cloth to get the inside part. Lubricate with grease or antisieze and reinstall.

 

I just fixed same problem on my truck (1999 Freightliner).
I noticed (in the dark) my brake lights staying on. Brakes dragging a slight bit for up to 40 seconds

I removed my brake air valve from firewall. Removed plunger (push cylinder), cleaned it.
The plunger had a crude on it that was preventing return to pedal when released.

My air pressure application gauge would return to zero from a leak in the system. I fixed that too?

I had extra time at home so yesterday I replaced the air valve also.
Quickest return to zero my application gauge has ever registered.