Hypac C 778 B won't move warm

Just got this machine, so don't know much about it. Took it for a spin and found that after it gets up to full temp, it slowly but surely goes from what seems to be a normal, strong pull to virtually no power in either direction. Steering remains normal feel, vibes also normal. Overall it seems in good shape, clean, fairly well cared for. When I say no power, let me expand that a bit. It's not exactly consistent. From a complete stop, it will try to move, but even on smooth, level pavement it will only occasionally start a little motion then take off. But if there is any resistance, like being in some softer material, it won't get going. I actually got it stuck.....in about 2" of asphalt millings and had to get a loader to nudge me out, but once I got moving it took off like a rabbit.

Any suggestions on what to look at first? It doesn't seem to be locked with brakes on, it just doesn't seem to have any power.

I've no books on it yet.

Sounds like hydraulic leakage .I don't know machine at all. I'd check return filters and suction strainers if they apply. Get a gauge in circuit watch pressure fall. What controls pump? Is it hydrostatic drive?

I agree that is what it sounds like. ight be just that simple. I hope it is, for a change. I don't believe there are any temperature compensating circuits to bypass when hot, for example. Overall, is seems it should be a very simple system. It is driven by two hydraulic motors, one for each axle. There is no mechanical differential or anything, since essentially it's a "2 Wheeled" vehicle". A simple flow divider balances the front/back best I can tell. I'll still hunting books. I remembered that the owner has a room full of manuals that might or might not have something on it. I don't know the entire hydraulic system at this point other to say that the ground speed is controlled electrically in fixed stages (analogous to gear ranges I suppose) by a simple "stick" that is on a shaft where there are two handles, one on each end for either of the operator's seats. From what little I drove it it does not appear to be a continuously variable flow control, but I could be wrong about that. It may be. Then there are two vibratory motors, but there doesn't seem to be any difference in them, hot or cold. They work, that's how I stuck it . Same with the steering. Simple single pivot point for the front roller. Again, no discernible difference in either effort to turn, or response. Until I can find a hydraulic schematic, I'm in the dark regarding any pressure relief valve that might be dropping the pressure to the propel motors under certain conditions. Guess a tee'd in gauge is next, unless I find a book on it, or someone has a better idea.

At least this one isn't committed to a customer's job for the owner at the moment. (That's a Scottish built Terex 30 ton artic with a ZF trans problem. It's on the front burner. It has 1rst, 3rd, and 6th, but no 2nd, 4th, or 6th.) Just his own 2 acre parking lot at this point. He just hired an ace blade man to flatten that (asphalt millings), and wants to be able to pack it after getting the drainage right. It does need that, but it's pretty wet from the recent rain, so I don't really think it's close to ready to pack yet. But this is still a job.

I suppose it's probably easiest to get directly into the motor circuit between the flow divider and each motor and start there with pressure readings. Just a cold V hot pressure may be very revealing. Or maybe better just between the main flow control valve and the flow divider since they should get enough power to move the thing even if only one wheel is pulling. Have to rip all the covers off and see how it's hosed up next I guess.

Kind of stuck out of it at the moment. We have a non-essential stay at home order in this city/county, and I'm thinking this doesn't qualify as exempt. So I'm catching up on small things in my shop, which I walk to from my home. It's out in the boons, so no risk of any exposure. It's a closed shop, no public.

Don't know how you got on but the symptoms match what was happening with my old Ingersoll tandem. Basically the variable piston pump which provides motion to the drive motors was well past it's use by date.(and couldn't be put back together again)

Flow case drains of pumps and motors if both in spec it's probably control circuit for pump control.

As it happens, I never touched that machine again. We had another machine, easier to fix, certainly quicker, so the guys got it going and this one was pushed aside for the time. I think eventually they got to it and replaced the pump and that was it, along with a good flush job. It’s somewhere in Alabama now I think.
Meanwhile I’m visiting L.A.