I have a '93 A35 with a brake issue that is driving me nuts! Have had brake issues every since I got this truck, low brake pressure light would go on/off. Finally a couple weeks ago I lost brakes altogether. No pressure at all. Pulled the brake pump and it looked wore out inside. So I got a new OEM brake pump that cost me 2K. Put the new pump on and all was good. Pressure light went out and had good brakes. Used the truck for two days and started having the brake light come on again ! Put a pressure gauge on the test port on the unloading valve and I can only get a max of 1000 psi. Ran through the whole brake bleeding procedure again and still no higher than 1000 psi. So then I tried to adjust the pressure according to the book and no matter how much I turn the set screw on the adjustment the pressure never raises or lowers! Could it be a bad unloading valve? How could I test this? I can't believe it's the pump as it is brand new. I also changed the filter when I installed the new pump. What am I missing ?
Could be the accumulators are giving up on you? have you tested them?
No, I don't have any way to test each accumulator. Would a bad accumulator(s) keep it from getting up to pressure? I would think it could still build pressure, just not maintain it when using the brakes.
A35 ? Straight, C, D , could you let us in on some more info. When in doubt, attach a pic.
A35 BM, older machine ser # A35V2177
The accumulators are what hold the pressure, see attached the last couple of lines describes the exact point you are at. you have tested pressure and can't get any higher, you have bled the system of air, and tried to increase the pressure through the adjustment, hopefully there is only 1 or 2 bad as they aren't cheap at £170 a piece to replace and there is possibly 14 on the machine
Here's a pic of schematic to get a better understanding of what works what.
I am curious if the foot valve is perhaps allowing oil to bypass to tank. Either that or possibly a damaged spool/seat in the pressure reducing valve.
Thanks guys! It just seems that if it is a bad accumulator(s) I should still see it get to the precharge pressure of the accumulators and it gets no where near that pressure. Odd that when adjusting the pressure on the unloading valve I see no movement in pressure at all. I'm leaning toward one of the valves in the pressure relief valve or unloading valve is bad or stuck.
That's why I suggested looking at the pressure reducing valve in the brake charging block./ unloading valve.
Looks like I'll be pulling those and tearing them apart. On the foot valve possibly leaking ..... do you think I could hear it in the valve if it was leaking ?
Maybe, it should also generate heat if it's allowing that much leakage. I'm leaning more towards the reducing valve. I will try to get you an exploded view today or tomorrow.
Hope this helps
Thanks ! I'm ordering parts. As long as I'm pulling it off and tearing it apart, I'll rebuild it.
Dang ... I am having a devil of a time getting any parts to rebuild the unloading valve! My Volvo parts guy says everything is obsolete but the O-rings. Anyone know a place that has parted out older Volvo trucks, especially an A35 !?
https://mwisales.com/ You could try these guys
Thanks, I'll give them a shout and see.
chickenminer what exactly are you looking for?
You could also contact your dealer and ask them to reach out to Lipperts. They buy up NOS dealer inventory. They might have what you're after
There were 3 repair kits for the unloader valve, but all seem to be no longer available through any source I have, the only option left that I can see looks to be a complete unit, but they aren't cheap. as FWF says there might be some NOS somewhere so try these, 11702321, 11702322 & 11702323
Thanks for the info guys! Yes, looking for a rebuild kit or the component parts. My Volvo dealer parts guy has been searching but can't find anything OEM or aftermarket.
Same this side of the pond, so it looks like there is nothing available except the new unit :-(
Just an update on this brake issue of mine. I got some parts last winter and this Spring I rebuilt the non-return valve. I could not find any parts to rebuild the unloading/relief valve or find a used valve. So I took the old one off and tore it apart. Nothing looked obviously bad in the valve, springs seemed fine and all components move freely. I did replace o-rings when I put it back together. Got it all back together and I still have the same issue of no brake pressure! Gauge still shows PSI will not go above 800 and as soon as I shut the machine down the pressure drops to zero rapidly. So I must have oil bypassing somewhere in the system. Sure wish I could find a used unloading/relief valve so I could be sure to rule that out.
Ok so I've gone back through this and realised that the parts drawing FWF supplied is for the wrong serial number range, the part number you want is 11061801. There doesn't seem to be a supersession between the two parts so I can only assume they are different, and there is no parts breakdown of the actual valve for that part number, but it also appears in an A35C so it opens up the chances of getting one used a little more. also something else to look at, there are a couple of none return valves in the valve block before it, could they be allowing pressure to seep back past them?
Thanks! I did replace the two non-return valves in the non-return valve block. Those were the only new parts I could get. The breakdown of the relief valve that FWF sent is correct. I used that for reference when I took mine apart. I fail to see, unless something in the relief valve is bad, why it will not build pressure beyond 800 PSI. I installed new filter in the pressure side but I could find no filter on the suction side and no mention of a screen in the oil tank.
Can you plumb a choke valve or steel ball valve in the hose that feeds the unloading valve with a gauge Teed in? I think you have to go back to the beginning. Establish that the pump is capable. I know it's new, but new doesn't always mean good.
I've been studying this back and forth. The non-returns are a non-issue while she's running. Did you change the oil when you replaced the pump? You said you did the filter but no mention of the oil.
I pulled the pump off and had the Volvo guys check it. They said it was fine so I have to take their word and rule out the pump as the issue. No, I did not change oil but I am going to dump it and make sure it's cleaned out. I did finally find a used unloading/relief valve that is suppose to be in good shape. We'll see if that solves the problem when it gets here.
Good luck, I know you've been fighting this for a while. A needle valve in-line to the unloading valve gauged will tell you without a doubt that the pump is up to task. Just don't deadhead it
Well I finally got my truck fixed! It was indeed a bad unloading/relief valve. Swapped for the used one I managed to find and my truck brake pressure is back up to normal. Thanks guys for all the help/suggestions! Now I'm back to hauling paydirt