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My late 99 F350 2WD DRW has an issue and despite GOOGLING and researching Ive found people with similar problems but not fixes. Here's my issue?

My truck after sitting still and not being touched for 3 weeks decides randomly to wee PSF on the ground last night. Ive traced it back to the PS tank overflowing through teh caps bleed hole. As I had had this random issue before (same scenario) I'd previously rigged up a clear tube to catch can arrangement a month ago on the reservoir cap to help pin point the leak. So last night it has leaked on the floor. (The truck has sat perfectly for 3 weeks prior and not touched and no mess was perfect over the weekend.) Lifted the hood as as previous Ive got fluid up the catch can hose as the reservoir is overflowing! For 3 weeks the PS fluid has been on the dip stick mark. The PS has previously been bled while running (turn wheels l/r etc) and levels checked when motor not running and all is perfect...
All I can think of that there must be a check valve somewhere that is deciding without prompting to allow fluid held under pressure to bleed back into the Res. Is this possible?? Where do I start looking??? Ive read on US sites of this issue but unable to find a reason as to why?
I l looked at reservoir this am and the fluid was up to the top lip (overflowing) so I started the truck and the fluid dropped to about the correct mark on the dip stick. Then with trucking running I turned the wheel left/right a few times and touched the brake etc and kept truck running and checked RES and you could only just see the fluid right at the bottom of the RES but essentially empty. Turned truck off and fluid stayed at the empty spot. Will be interesting to see if it comes back up over the next few days while not being touched...................................
Could there be a check valve opening randomly that shouldnt in the feed line or something???

How can this happen?

Thanks heaps in advance Matt
 

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Ah ha! My truck did the same thing a couple months ago. After sitting for a week or two (I don't recall), all of a sudden there was a puddle under my truck, coming out of the power steering resevoir.

SO, I searched and came up with nothing.

However, knowing how the hydro-boost system works, I've concluded that the "leak" is actually the accumulator in the hydro-boost system slowly leaking down over time. There's a nitrogen charged reservoir that accumulates fluid and keeps it under pressure, so that you still have power assist to the brakes if the engine dies. IF the valve that holds that pressure is worn at all (mine's only 20 years old and only has like what, 285k miles on it), that will force fluid back to the power steering pump reservoir and potentially out the cap.

Again, I haven't found much on the 'net about this but looking at how that system functions, that's my best guess.

Clay
 

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Hi Clay that does make sense and I appreciate the reply. My truck is a late 99 model but has only done 80000ks though I doubt that means much. What Ive now done is a complete flush of the PS system. If post this I get a message on my garage floor than I'll replace the accumulator. Is this what you did and if so job difficulty? Appreciate the heads up. Matt
 

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Hey Matt,

Well, I haven't actually gotten in there to replace anything yet. I've actually been working this entire time during the pandemic; was working from home 100% of the time when this started, and now I've been driving the truck every 3 days or so for a while. I haven't seen any more fluid coming out yet which is good. I am not sure how much the accumulator will hold.

Worst case I bet replacement of the hydro booster (behind the master cylinder) would include a new accumulator. Not sure I'd want to mess with replacing any of the internals, as you may end up chasing problems on through the system that way.

BTW, have you seen the oil absorbing mats? They do work well!

Clay
 
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