Ronmar
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How did you know it was the park brake? Did the park and emer brake lights illuminate in the indicator panel?
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Adjusting Load Sensing Valve on M1078
- Thread starter KGBruce
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spankybear
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Reading the TM it says to hook up a pressure transducer to the brake can and then hook up to the STE/ICE. Are they just using the STE/ICE as a very expensive pressure gauge? I think my load sensing valve isn't adjusted right. I want to check but don't have a STE/ICE thingy
Ronmar
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That is what it sounds like. Did the procedure have you setting it for a particular pressure? unless they have some arbitrary pressure spec in mind, that doesn’t make a lot of sense for setting the proportioning valve. I mean the point of the valve is to byas the brakes for varying weight in the back, and with the different configuration from front to rear, the only way that makes sense to adjust it is the way described above, testing wheel lockup on a gravel road. That is the way I have always setup toyota’s. Before I did this however I would confirm the brake shoe clearance is the same on all 4 wheels...
spankybear
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Yes they have a table
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Ronmar
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Interesting, I would assume this would be with an empty truck, and not one with an extra ton or so of cargo in the bed, even though the initial set-up for the procedure doesn’t stipulate this...
note the pressure diff between the 1078 and the 1080(cab and chassis only, about 2400# lighter) . I am surprised there isn’t a pressure diff between the 1078 and the 1079, as the 1079 weighs 1100# more and that weight is all over the rear axle...In answer to your initial question, yes, the STE is being used as a fancy pressure gauge. And you can confirm that the pressure you set is correct on a dirt road
spankybear
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I was having an issue in snow... with my foot on the brake and letting up the rear wheels started to spin before the front brakes released. The was causing issues.
Ronmar
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Interesting. Because of the proportioning valve, and according to that chart, the pressure is a lot greater(3X?) on the front than on the rear, but the rear is twice the chamber volume, so it might take a little longer to release the front... did you lock them up while applying brakes? Which locked up first while applying brakes?
spankybear
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Don't know where you got that the rear pressure is greater than the front with what I posted. It only talks about the rear service air.
The air will be less to the rear wheels (no load in the bed) and the brake service chambers should be same volume front/rear. I believe they take the same diaphragm. The reason the rear "cans" look bigger is because they are "maxi brakes" or "spring brakes" http://www.rogerstrailers.com/contact/glossary/maxi-brakes
No I did not lock up the brakes. I just noticed in the mirrors when I started to let or on the brake peddle the rear wheels started to move and spin in the snow. That tells me that the load sensing valve maybe not adjusted right. Anyway it looks easy to test if you can used a pressure gauge. You just need to tap the service line on the left side rear brake can.
Ronmar
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I said the pressure was greater in the front, as it receives full system air passed by the treadle at full depression. The diaphragms are the same size, but there are 4 actuators to fill in the back vis the 2 actuators in the front... IE: if the cans are equal size, 2 cans at 3XPSI = more air volume than 4 cans at 1/3 PSI...
locking up on ice/snow will also tell you how the front/rear bias is adjusted...
spankybear
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I do not know this to be true that 100% of system pressure is applied to the front. Do you know where in the TM it talks about this?
Ronmar
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I am not sure it is listed in the manual, but I have measured service pressure and it was over 100PSI when I did it. But it was my grandson stepping on the pedal and I am not sure exactly how hard he was stepping on it and what the primary/secondary pressure gauge was reading at that instant as we had just completed a few different stopping pressure tests. Like using just enough pedal to bring the truck to a stop at idle, It took around 75PSI in 1st and around 50PSI in 2nd... that pressure was measured at the rear service gladhand...
The treadle valve is a mechanical regulator, but instead of a screw and knob to set pressure like on your shop air compressor, It uses a lever and pushrod. the harder you push on the lever, the more output pressure you get, up to input/tank pressure. The secondary side is connected directly to the front relay/QR valve and on to the cans.