B21 HST filter blows apart

[princekp]

Hello, my B21 (probably late 90's- early 2000's?? model with backhoe and loader) keeps blowing the HST filter apart. So far, it has destroyed at least 4 expensive filters. I have changed the oil and all filters (using genuine Kubota filters but generic oil that is supposed to be "approved")and even removed and cleaned the inside of the hyd. tank-it had very little gunk in it. While I had the tank off, I blew air through the lines- everything was clear. I inspected where the return line from the filter enters the trans- it looked good, and I blew air through it- it was clear. Also, i should add, this is happening at about 65 -75 outside air temp, so I don't think its a cold oil issue. The filter blows after the tractor runs just long enough to give me a glimpse of hope that the problem is fixed- anywhere from 2 - 10 minutes. It doesn't seem to matter if it is in neutral or in any gear. I don't think that it has ever blown while driving, but I haven't driven it enough since this problem started to say for sure. Everything seems to function properly, just keep blowing the filter. PLEASE help, I am at a loss as to what to do next.

Thanks in Advance.

 

[Eric McCarthy]

Does it blow more like an explosion and is busted wide open or does it blow off the threads and wont stay on? If its blowing off the threads where the filter screws on, could be the threads are striped

 

[Stumpy]

Agreed. Otherwise it's likely something is wrong with the hydro and you're getting way too much pressure at the filter. A service manual would be a good investment if you're going to try digging through this yourself because it can get a little deep.

First step would be confirming you've got an over pressure problem by testing the pressure in each circuit. The manual will outline the procedure and you'll need a high pressure gauge. You could also skip right to the next step which would be removing and checking the pressure relief valves. There are three of them, a low pressure valve on the charge pump behind the input shaft, and a forward and backward high pressure valve on the transmission block. Gunked up ports and broken springs or orings would be the thing to look for. I don't know how it's plumbed exactly so I won't try and guess which valve to look at first, hopefully the service manual has a hydraulic diagram.

Goto Kubota's website, punch in your tractor, and look up HST under transmission if you want to get a rough picture of what you're looking it.

 

[ssls6]

The housing that holds a spin on filter will often contain a bypass valve (usually a ball and spring). The purpose is to bypass fluid when it is very viscous like at start up so the filter doesn't blow apart. I'm new to Kubota but I have seen this problem on other engines. It's the first thing I would look at or ask the dealer about.

 

[princekp]

The filter itself is what is failing. It comes apart at the seam. I don't think the filter housing has a bypass- it does have what I take to be an air bleeder (I could be wrong) which attaches to a flimsy rubber hose and snakes around to the bell housing. This aparatus has such a tiny orfice, I doubt that hydro fluid could pass through very well. The rubber hose can be pulled off by hand, so I doubt that it could withstand any pressure and it doesn't go back to the tank anyway. I really appreciate the ideas.

I think the relief valves (thx Stumpy) will be my next step. Hopefully, we can figure this thing out.

 

[ssls6]

The filter itself is what is failing. It comes apart at the seam. I don't think the filter housing has a bypass- it does have what I take to be an air bleeder (I could be wrong) which attaches to a flimsy rubber hose and snakes around to the bell housing. This aparatus has such a tiny orfice, I doubt that hydro fluid could pass through very well. The rubber hose can be pulled off by hand, so I doubt that it could withstand any pressure and it doesn't go back to the tank anyway. I really appreciate the ideas.

I think the relief valves (thx Stumpy) will be my next step. Hopefully, we can figure this thing out.

This is what it looks like on a chevy.

[/IMG]

It's not in the filter but in the housing that the filter screws to.