Backpressure issues w/Hydratech

[Hydratech®]

Great to hear that we are making positive diagnostic progress. What we have proven out here is that the brake assist unit IS indeed fine, and was self applying due to the back pressure on the PS system low pressure return line circuit. Any back pressure higher than 3+ PSI on the hydraulic brake assist unit's low pressure return line will induce brake self apply. The problem that we must now pursue further is where and why this back pressure exists in your original plumbing arrangement. You should ideally see only 1-2 PSI of pressure in the low pressure return line circuit(s) of a PS system. What would be neat and highly informative would be to have you remove the cap on the T fitting you currently have installed to gauge what back pressure levels we are dealing with. Connecting up a 0-20 PSI gauge onto the T fitting would speak volumes to see exactly how high the back pressure is...

There are 4 thoughts that come to mind:

* Restriction in the PS cooler (even though it looks to be a nice design of free flowing nature)

* Restriction in the plastic injection molded low pressure return line nipple (in the PS pump reservoir housing)(possibly molded incorrectly with bottle necked inner diameter causing a choke point)

* Possible collapsed internal Teflon hose liner in one of the hoses (very rare, but nevertheless possible) (or an obstruction of foreign nature clogging free flow)

* Excessive PS pump flow (optimum target is 2.2-2.6 GPM (gallons per minute of pump flow) (some truck spec pumps can flow up to 3.5-3.8 GPM)


If I was there working with you, the next move we would make would be to restore everything back the way it was initially plumbed (putting the original braided stainless low pressure return line you had back in). I would then take the rubber test hose you have on hand and use it to bypass the braided hose running from the PS cooler back to the PS pump reservoir. This would tell us if my suspicion is correct in having a bottleneck to flow at the PS pump reservoir low pressure return line nipple (injection molded in the plastic PS pump housing). IF you run from the cooler to the "mouth" of the PS pump reservoir with your bypass hose that you have on hand, and experience no brake self apply when you start the engine, this tells us that your restriction is NOT in the placement of the T fitting (before the cooler) AND it is NOT the cooler causing the restriction to flow.



IF all is well with the bypass hose in place as spelled out above, then we could take a look at possibly drilling out the inner diameter of the low pressure return line nipple molded into the PS pump housing JUST A LITTLE BIT. Can't do much though, as you don't want to weaken the plastic nipple molded into the reservoir as it may then just snap off when least expected. IF the same problem exists with the bypass hose connected from the output of the cooler to the mouth of the PS pump reservoir / fill cap opening, then we could suspect the cooler as the restriction to flow (and would then bypass it completely to test).

Summary? You do have -6 AN lines from what I see (not -4 AN) Increasing line size will likely not help. I do suggest that you try to determine where the bottleneck causing back pressure exists. Placing the T fitting AFTER the PS cooler will only help IF the cooler is indeed the point of restriction. IF the restriction point is indeed a bottleneck at the low pressure return line nipple at the PS pump housing, moving the T fitting will NOT help one bit. If you don't want to get into all of the diagnostics, you could install the AN-6 bulkhead adapter into the side of your existing PS pump reservoir to run a second dedicated low pressure return line from the brake assist unit straight to the PS reservoir as shown in the links in the previous posts. *OR* slow the pump flow down with the installation of a replacement PS pump output fitting (with a 1.125" orifice size as supplied by DSE or PSC discussed in the posts above). Slowing down the overall pump flow with the simple / inexpensive swap out of the PS pump output fitting could very well take care of business for you, as less flow from the pump = less fluid being bottle necked up through any points in the low pressure return line plumbing.

Please feel free to call Jim to discuss further.