Vintage Horticultural and Garden Machinery Club

I had same problem years ago. There was a pressure relief valve in a special fitting which relieved the tank pressure from the cylinder. When I opened it up the little stopper was covered in gunk. Cleaned it up and gave the supporting spring a slight stretch and it ran fine again.

There’s also a little valve over the piston that moves up and down with air movement (usually made of thin stainless steel…it could be gunked up too.