Hydraulic steering get stiff after 20 min driving - jaguar 420

I own a 1968 Jaguar 420 which I have refurbished over the last 3 years.
In the recent days I have started to drive it
At the beginning of the ride the steering wheel feels really soft and light, but when about 20 minutes pass it starts to get harden and slowly loses all its hydraulic power at all.
This phenomenon is repeated on it self
Every time the car cools down a bit the steering wheel becomes soft again, and after 10-20 min its get hard(stiff) again.
It seems that when the oil heats up, it does not function properly.
the hydraulic pump is from a saginaw company and has had gaskets replaced by me,
There is no leaks at all and the oil level is normal.
I use Castrol ATF oil
Any idea?
Thank you

If you have the Saginaw pump, then you probably have an Adwest Marles-variamatic steering box ?

There isnt much in the pump, but I think there is a valve…look in the FSM

The steering box itself is quite complex, and may be at fault

I have seen examples where the power piston or bore inside is compromised

The pump diagnosis in the FSM involves a pressure gauge, but it runs up over 1000psi

ATF is specified

I can only suggest you run through the FSM diagnostic tree

I had new hoses made for mine, its possible you have an internal wall collapse

why did you rebuild the pump ?

Hi Tony.
yes, i have an Adwest Marles-varimatic box
I was rebuild the pump as part of restoring all the car.
like you,I know there is not much in the pump, but the phenomenon that the steering wheel becomes stiff after the car heats up is more suspicious of the pump than the steering box. am I right?
I will try to find a pressure gauge and measure the pressure in the pump when the oil is cold and also when its hot

I have a 1967 Mark 2 fitted new from the factory with the Marles varamatic power steering system. The problem you describe may be caused by an old filter in the fluid reservoir. I encountered the stiffness you describe a year ago. A competent (and honest) mechanic suggested I remove the filter from the reservoir and see if that would make a difference. I ordered a replacement filter from SNG and the problem was solved. Looking over the Mark 2 maintenance chart which came with the car, I noticed it calls for replacement of the power steering filter every 20,000 miles. Remove the filter from yours and see how the pump behaves without it.

do you know if it worked ok before you rebuilt it ?

there is a valve, highlighted as a possible fault

the pressure gauge arrangement shown in the FSM looks a bit of work, and I know the HP hose fittings are problematic, I had mine custom made

You may have to custom make a gauge

do not overlook a possible loose belt

there should be no filter that I know of in your combo (Saginaw PS pump)

I would definitely suspect the HP hose if its original, the LP one is easy to replace

that sort of issue is a known problem on the front brake hoses of these and other vehicles, internal swelling takes place