Scubapro Mk 20 Pressure Creep

[Isaac-1]

I have a Mk 20 with what seems to me an unusual IP creep problem, I am not an expert on the Mk 20, but have rebuilt a good number of Mk 2,5,7 & 10's, so before guessing I thought I would ask here. Here is the situation, upon initial pressurization (I have only tested with a half full tank so far) the IP comes up to 135 psi instantly, then creeps up to 150 after another 45 or so seconds. In another 5 minutes it will creep up to 155 psi, there is no further creep or second stage leak after this. While purging the second stage IP drops to 125 psi, then instantly returns to 135 psi when purge is released, it then creeps back up to 150 psi much faster than the initial 45 seconds (more like 15-20 seconds), then on to 155 psi after another 5 minutes. I suspect there are multple issues at play here, but not knowing the Mk 20 very well makes me wonder if there is some Mk 20 specific issue at play.

thanks Ike

 

[DA Aquamaster]

It is a Mk 20 specific problem. The Mk 20 had three different piston designs depending on when it was made.

1. The early piston was a stainless steel piston with a knife edge seating surface. It worked great.

2. The mid production Mk 20 used a brass tipped piston that had a rounded seating edge. They were notorious for having a mushy lockup and IP creep and in fact have a much looser set of allowable specifications for IP creep. A "normal" piston is expected to lock up very solidly with little or no creep in the first 5 seconds and none after that. The Mk 20 was allowed about 5 psi of creep initially and a few more psi over the next minute. I pretty much replace them on site as it is a "when they fail" rather than "if they fail" issue.

3. The late production Mk 20 used the same composite piston used in the current Mk 25. It also has a knife edge and locks up very solidly. It is a direct replacement part and the dealer cost is about $25. I it also replaced free under warranty if the IP will not lock up.

If you do not have a brass tipped piston, I suspect the seating surface is damaged so piston replacement is still the most likely fix.

 

[redacted]

While the brass piston is a PITA, I have had some luck cleaning them up, lubing the seating surfaces, and restoring reasonable performance for a while. I have replaced my brass pistons but keep them on hand in case one of the new composite pistons fails.

 

[wmspdi]

I had the same creep problem with a MK20 (it also had a mid production piston described above). Piston replacement solved the problem. I would go with DA Aquamaster's recommendation every time.

 

[halocline]

I had some luck with a friend's brass tipped MK20 piston by carefully polishing the the seating surface of the piston with micromesh. I basically lap-sanded the flat "edge" with 2400 grit micromesh, then hand polished the whole edge with 3600 grit. After several minutes of that, there were no visible flaws in the surface looking with a 16X loupe. It holds IP now, but I also ordered a MK25 piston for the inevitable, and as soon as I get it I'll put it in.

I just rebuilt an early MK20 here in Roatan that had a history of freeflows through both 2nd stages well into a dive; a sure sign of gradual IP creep. I expected to find the middle generation piston when I took it apart, but it was the steel knife edge, although not nearly as sharp a knife edge as the MK5/10. There was no available MK25 piston so I tried the same trick, except I only had 1200 grit, and it seems to be working well. I was REALLY careful with the sanding, and the micromesh is a very uniform and soft grit. It really just cleans the surface. So far so good, but I told my friend who owns the reg if the IP starts creeping again it's time for a new piston.

 

[Isaac-1]

just wanted to say thanks to everyone