7020 steering problem

[peterbilt07]

When driving my 7020 I had problem that I never noticed before.I had tractor in gear with clutch pushed in waiting on traffic and within 15 seconds I had no steering and main hydrolic pump started cavitating.I realize my resevoir drained and had no hydrolic fluid. I took it out of gear let clutch out and almost immediately had steering back.I Know its not running the rear end pump with clutch pushed in,but it should not loose steering that fast.Main pump has been rebuilt and have no external leaks.I was told it could be pto valve or steering valve causing problem?When clutch is out everything works perfect.I am shure it is internal high pressure leak and is returning fluid to fast but what is a good place to start? Thanks.

 

[tim s]

Check and see if one of your components are heating up a bit quicker than normal, high pressure leaks cause heat, if that don't point you in a good direction start isolating components one at a time to see if that will help locate it,steer valve,brake valve,rockshaft,,the rockshaft would be a good place to start..

 

[peterbilt07]

Tim tractor is a bareback.I have been restoring tractor since feb.The outlet control valves in cab was leaking bad when I bought tractor so i had them rebuilt and tested by local JD dealer.Other than the valves I had no other leaks that I could detect.After installing valves back in tractor main imput shaft on front pump started leaking so I had pump resealed and rebuild kit installed.What is easiest way to isolate components if not heating up?

 

[tim s]

I keep an assortment of JIC caps and plugs for isolating things, do one at a time so you have a better idea which one to look at, like for the brake valve cap the pressure 'in' line and start up the tractor and see if the failure still happens the same way, then the steer valve, for this you will need to hook a remote cylinder into the back to see if your loosing pressure. A worn trans pump will act this way too, but it sounds like it recovers fairly quick when your let the clutch out, so I'm thinking it may be okay.

 

[peterbilt07]

Tim,after you gave me advice I also talked with my local JD dealer that did my valves [the service manager was there when they sold 7020s new]told me exactly what you said,start tractor and run it at 1500 rpm walk away for 15 min shut it off and feel what could get hot if it was leaking buy on high pressure.Everything was cold except hydrolic relief valve in front of cowl and return line under dash.The relief valve has a outlet that dumps back to return line that would make line hot if it was bad.Valve part #AT30373.Have you seen one bad or could I be missing something?Valve is $245.00.I am just covering all bases.

 

[Tx Jim]

Before I'd spend $245 on that valve I'd either shim it up some more or block it off to check to see if it actually was the problem.

W19898 Shim AR 0.25 MM (.010")
W19897 Shim AR 0.51 MM (.020")
W20088 Washer AR

 

[tim s]

That sounds like the place to start, I haven't been into one of those valves for a while, can't remember how they are, but I would remove it and take it apart, you may be able to clean it up and re-furbish it...there are very few 7020/7520's in service around here now..