"when i go straight, OK , when I turn right , OK too-very smooth and precise. But when I turn left the steering wheel needs a bit more effort and the steering is less precise and smooth and it's "faster " than when I turn right.I mean bends at 40-50mph

after replacing the PS control valve, the steering has becomeharder than before"

Hi Aldo, I agree with 73shark, it sounds like you have to balance the new control valve. Go back to where batsey80 said and look under " Control valve replace and balance instructions".
The picture you posted is the steering box, the control valve is under your car. PG

 

when we fitted the new PS control valve ( a Lonestar Caliper) seemed that no centering was needed, since the wheels were not moving by themselves.

At the shop, I wanted to check the control valve centering with the PS piston detached from frame, as suggested by GM instructions, but with car jacked up , the engine fan was touching the fan shroud due to chassis warping...

So , may I adjust the control valve centering with the car on the floor and without detaching the PS piston? turning the nut just a bit and trying the steering wheel turning effort?I'll do this step by step until I have a more balanced steering wheel...

 

You are very close to having it centered 100%. I did mine in the driveway with the front wheels on ramps. I took off the rod from the piston and slowly turned the adjustment nut left than right than split the difference. Sounds like turning the nut 5* from where you are now will do it. PG.

 

If jacking your car up flexes the frame enuf to cause the fan to hit the shroud, then you might want to check your motor mounts (maybe you already have, I can't remember) or look for some places on the frame that have serious rust as that is a problem with some that have seen a lot of road salt and weren't flushed out.

 

PG . I'll try to balance the valve asap. I hope to solve the problem.good idea the ramps...

73shark . the car was totally jacked up, not the front only. look at pic. the nose isa bitdown due to nose weight. the clearance between the fan and the shroud at the top is sosmall that I'm not surprised of fan hitting the shroud on certain conditions. Anyway I have only surface rust underside ( california car).Possibly the motor mounts... Anyway when I rev the engine I see little-usual movement of it. How to check motor mounts?

[IMG]local://upfiles/6949/C246C7F1A0CB47A9A821FDC1D4942AB7.jpg[/IMG]

 

Look at the gap between the bottom of the fender and the door, it closed up. The pic of your car on the ground shows a normal gap.

When my left motor mount broke the left side of the motor would lift up and the fan hit the shroud. To test yours power brake it in 1st gear and see if the motor moves. To test the right side power brake it in rev. PG.

 

PG,

the gap at door seems ok to me too. A friend's 79 vette has a wider gap than mine, for example...

I already made a braking test both forward and in rev ( for other reasons: after replacing front coils and shocks). I heard no strange noises from the fan.

I suppose that the reduced clearance between fan and shroud, it's possibly due to 2 things

- my engine it's not the stock one , but was replaced about 20 years ago, I suppose
-the radiator seems replaced too.

So possibly , when reassembling the things , not so much care has been took to perfectly fit the parts.
anyway, I'll post a pair of pics of fan and motor mounts soon, so you can judge the situation

 

PG , some pics , but not the motor mounts : too hard to take good pics

Anyway , I think that the shroud is fixed too low, simply. check pics

Ah... look at the second pic. My fan's shroud has a "door" underside. My friend's 79 doesn't have it and can't find this thing on the assembly manual. fresh air to radiator and engine I suppose...Indeed my car has no overheating problems ( till now...)

[IMG]local://upfiles/6949/4C4C9AC0BA6D4826A301097D9464FCCE.jpg[/IMG]

[IMG]local://upfiles/6949/5D86EB613DBE448E86B6394574EAAA12.jpg[/IMG]

[IMG]local://upfiles/6949/6658D30AA33E48EFAACD68EEFA0CF4A7.jpg[/IMG]

 

The fan should have pretty equal clearance all the way around the shroud. So the clearance on the sides should be the same as on top. If it's closer on top, then something is not right.

 

Hi Aldo, I had a chance to look a my fan clearance today, I have more at the top, more still at the left and less at the right. My opionon, just live with it. Check your transmission mount and if that is ok, than so be it. Your fan shroud may be off a little, I have bolts where you show a screw. If you want pic's let me know.
I have seen post's where someone finds out months or years after the purchasethat there car is 1/2 inch lower or higher on one side or the other, so what! Our cars are up in years and they were not made perfect from GM. Enjoy your ride! PG.