On presses with a Stauff recirc filter, the check valve inside the filter has been known to fail and drop the cotter pin, spring and washer into the oil tank. The spring can get sucked up the goose neck into the prefill valve where it holds the valve open.
To determine if you potentially have the problem of the failed check valve, open the filter canister. Look at the check valve. If the spring, washer and cotter pin are missing, you should search for the spring immediately.
Despite the command to close, the prefill valve can stay open with the spring stuck in the valve seat.
The Stauff filter check valve can be modified to replace the cotter pin and use a bolt to secure the check valve. Loctite is needed to lock the nut on the shaft.
The check valve assembly comes new with cotter pin to secure the spring and check valve washer. The cotter pin can shear due to a clogged filter.
This picture shows the bolt replacement for the clevis pin and cotter pin original.
The spring height is very important to match with the original height using the clevis and cotter pin.
View from the top of the filter assembly with a good bypass (check) valve installed.
This is the original clevis and cotter pin provided by Stauff
