D-8 15-A hydraulic brake booster

Hello everyone, my D-8 15A has been setting since last summer. I notice the left brake pedal has 0 resistance when you push it in while it’s not running, and the linkage Rodney doesn’t move at all. The right side does and you can feel and see the linkage rod activate the brake heat fluid inside moving. I fired it up and checked to make sure the oil is full on the dipstick and it was. After pumping the pedal several times I have a small amount or resistance the last inch or so before it’s all the way to the end of its stroke. I’ve never had to work on the booster before. The book shows a activating valve I’m assuming it shifts-moves to feed oil to either side? Could this be stuck? Just thought I’d ask before I start digging into it.
Thanks.