I own about 3 of these multi-purpose water sprayers. Two of them I use primarily to wash my car in my apartment complex due to leasing policies and restrictions, however, they recently began to under perform and lose pressure despite additional pumping.
I saw that the OEM gaskets were not properly sealing the end of the piston cylinder housing, so I decided to design and manufacture my own using 3D printing technology with TPU (Thermoplastic Polyurethane) material which is perfect for this application.
Images below show the before and after the fix. I decided to increase the thickness of this gasket to ensure more layers adhere together to get as much of a water tight seal as possible.