Hi,
You hit the nail on the head. This is the information Speakman sent:
This is our S-1735. This unit is obsolete and parts are limited. I would recommend that the unit be opened and cleaned. I would remove the pistons and clean them with a cheese cloth. I would also clean the chambers that they go in with a cheese cloth as well. There are also some washers that can be replaced. I would double up on them they are thin. If this does not restore the unit to working order you will need to replace the valve block assembly.
G05-0218-MO (valve block assembly)
RPG45-0011 (seals & washers) use this number G99-0073-MO
RPG15-0063 (yoke & plungers) discontinued
RPG07-0004 (piston cap)