Going to need a picture for that one. Not familiar with a T51 pump.

Hello , changed the shaft seals last year . Got everything from Cat . When you order seal 5F4721 they will give you 3H 1397 . The number has changed because of improvement or so .
The Netherlands , johan 7

Well, obviously 200 psi isn't going to move much, but you could have a clogged filter or screen assembly or a bad valve, bad cylinder packing etc depending on how you're checking the pressure. Or your pump could be worn out or something stuck in it.

Like OM says, need some pictures of that one.

Sudden failures are possible. I've seen gear pumps that were apparently fine when a machine was shut down and wouldn't put out enough pressure to lift anything when the machine was restarted. (CAT pumps were all vane types, but the same applies). Probably the most common cause of sudden failure is running dry.

Hi Skip,
be aware that the pistons in the hyd. cyls. have relief valves built into them to stop pressure spikes when ever the pistons have travelled fully in or out.
If you bottom the cyls. to test the Hyd. Control main pressure relief valve funtion, you will only get a low pressure reading due to the oil flowing thru these piston R.V's.
The system must be stalled by outside means, ie, blade cutting edge dug into the ground under a rock or tree root or blade jamed under something heavy enough to blow the valve.

As a result of this issue, at The Dealer, these main relief valves were bench tested on a special test plate drilled and tapped to mount the valve and connect to a 6F25 Cat Service Press Pump--Cats recommended way of doing it.
Hope this helps.
Cheers,
Eddie B.

I have a similar problem. I have a D6 which I think is about 1954/ I have the serial number somewhere, but not at hand.

My problem is that the pump does not seem to be pumping. To get the blade to lift, I have to hold the control in the UP position for a long time and it inches up slowly. I do not have any user manual or drawing or picture of the hydraulic system, which makes it hard to know what to do on it.

I assumed that the tank in front was the hydraulic reservoir, and I checked it and it is full of oil, so I assume it is not a lack of hydraulic oil that is keeping it from lifting the blade. It worked fine before.

So, I assume that for some reason that the pump is not pumping like it should be. So, what is my next step? Should I remove the pump and reservoir as was done above. If so, what do I do then? Can the pump be rebuilt? Are used replacements available? I would appreciate any help on this. Otherwise the tractor runs great.

jim nicholson

The D6 most likely uses the #46 hydraulic Control Unit. Based on others with attempts to rebuild/repair these units, especially the pumps, findings are that parts are no longer available and there is no longer anyone offering exchange rebuilds so your pretty much limited to used if you can find them. Some of the other non-pump items are still available from Cat.
The first thing to check would be the main relief valve (in the tank). The spring used has been known to break resulting in little or no output. In general these are pretty stout built units and given a reasonable amount of care they last a long time.

Hello , seal with part number 5F4721 is replaced by part number 3H 1397 and is available at any Cat dealer for about $ 15 .
The Netherlands , johan7

"Old wrote:

The D6 most likely uses the #46 hydraulic Control Unit. Based on others with attempts to rebuild/repair these units, especially the pumps, findings are that parts are no longer available and there is no longer anyone offering exchange rebuilds so your pretty much limited to used if you can find them. Some of the other non-pump items are still available from Cat.
The first thing to check would be the main relief valve (in the tank). The spring used has been known to break resulting in little or no output. In general these are pretty stout built units and given a reasonable amount of care they last a long time.

OK, thanks. That would make sense about the relief valve, because I am able to get it to work at a reduced speed by holding the control handle in the up position.

Is this relief valve accessible without having to remove the oil reservoir and pump? Please let me know on that?

The reservoir and pump do not have to be removed. Access is available by removing the reservoir cover with the rest of the unit in place. There are other sources of leakage possible but sudden failure makes the relief valve spring a good candidate to check. Last I heard they were still available from Cat.

I have a relief valve/ spring if need be. It was removed from a unit that was savaged(destroyed) by a junkyard torch. I was able to keep some usable internal parts.