Brake pedal SLOW to return Any ideas??

[chev-obsession]

So I just got my 69 c10 truck running that has been sitting for 20 years or so. I replaced the master cylinder with a new one for a 72 model and converted to disc brakes in front. I have the original 69 brake booster and brackets. When I press the brake pedal it is VERY slow to return! Even with the engine running.

No broken springs in rear
New rubber hoses up front
No kinks in brake lines
This issue seemed to be prevalent before doing any work and when it was pulled from the field.

Any help is appreciated

[Accelo]

Your Pedal check valves are damaged.
Power brake sluggish return is almost always the relief valve built into the sealing ring at the plunger input failing to let in fresh air and you're fighting a vacuum from the other side.
It allows the diaphragm spring to overcome the engine vacuum and return the brake pedal to the normal position.
In all likelihood you will need to replace or rebuild the booster.

[pjmoreland]

The brake pedal return spring was missing on my truck when I first bought it.