We have an PTV waterjet which is assembeld with H2O parts(now KMT).
Problem is on the pump, normal working oil pressure is 200-220 bar and outgoing water pressure should be 3800 bar but as this problem appeared there is 2800-3000 bar water pressure. At first I thought that there is lekaing somwhere but there is no leaking anywhere. PTV says that they never heared for this problem and they don't know what to do so I'm asking you if you have any sugestion.

Sounds like it is either time for a seal change or there is an issue with the check valve.  Perhaps a bad seat or a hairline crack in it.  With any of these cases you would see oil pressure at normal, no signs of leakage, and loss of output pressure.

I've checked check valves and still nothing, I have only 3,100 Bars. I'm using CHUKAR parts, are they good?

I don't have any expeirence with Chukar parts and don't really have any experience with their quality.  Generally speaking, I would stick with OEM parts for pumps.  It makes less difference on cutting head parts, but at least with Flow, I know that there are alot of metallurgical processes on their pump parts that often get skipped when others make the same parts.

One other thing you may want to check is that the bleed down valve in the pump isn't bleeding water during cutting.  There should be two water drain lines coming out of the pump that usually for to a drain, one for cooling and the other to relieve pressure.  Check the one that relieves the pressure.  It shouldn't have water coming out of it during cutting.  If it does, it may need to be rebuilt.  I have less experience with these pumps, so it may also have an on/off valve in the pump that would perform the same function.

there are many reasons for pressure loss except leaking, like diamond becoming old,high pressure seal old and etc. You need to check these fittings

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Problem Is solved. Problem was in the liner filter http://www.ptv.cz/ptv/jnp/en/produkty/nahradni_dily/Sestava_linioveho_filtru/index.html , it was never changed in 5000 hours of working because no one told us that this filter could be the problem.