Imagine - a client brings you a Kawaski K3V112DT (a technical brochure on these pumps here)
pump to overhaul. The main complaint is multiple oil leaks, aside that
the excavator is performing "better than new". You disassemble the pump
and, indeed, find lightly marked valve plate and the swashplate (pic1), and almost intact rotary group, with minimal wear of pistons, or the barrel.
This leads you to assuming that the oil used in the hydraulic system is
kept in excellent condition, and, as the client has no complaint
whatsoever about the performance of the pump, all you need to do is
throw in a seal-kit, top it with a bench-test and send the unit to the
client, right? Wrong!!!
It is not uncommon for a technician, after seeing
several pump parts in excellent condition during disassembly, to assume
that the rest of the unit is OK too. In 90 percent of cases he will be
right, but the resting 10 percent when he is wrong will hurt like
hell... That is why it is important to dismount a pump fully and examine it in detail
even when the parts don't present any damage or excessive wear and even
when the client assures you that the pump is working fine!
These pictures are a good example of how important
this is. As you can see, the rotary group parts present normal wear
(minimal) and are in perfect condition. However, when you take out the
boost pressure check valves, which are inside the middle section of the
pump (key n. 986 on the drawing), you will see that they are severely damaged and are about to break. You can see how the seat is supposed to look like here.
The assumption that the pump is in perfect condition can easily lead a
mechanic to skip checking the valves and cause a premature failure.
The best way to approach a pump's overhaul is by double-checking everything no matter what!
Read, also, the article on pump boost pressure checks and shuttles here.