cat 938g series 2 brakes

[mobile-tech]

maybe 50 psi difference the presssure slowly creeped up to 400 plus both hit 300 the creeped up
ill get everything together and preform the test you posted and report back , thnks

 

[Nige]

Make sure you do both of the test procedures and then compare the results.

It would be especially interesting to see the brake accumulator precharge pressure even though the number you get from the test is only approximate.

 

[mobile-tech]

ok test 1 : start engine aand idle pressue comes up then falls alittle to 1750 cuts in runs til 2050-2100 psi cuts out
7 pumps down til i hit 1350psi brake light turns on , countinune pumping down til gauge drops suddenly 16 additional pumps
text 2: start and run at high idle the slight pause in pressure before quick rise 700-1000psi its raises prettty fast to cut out pressure 2100psi and holds aply brakes 3 to 4 pumps hits 1750 psi cuts in , repeatedly apply the brakes cuts in ans out per spec

 

[Nige]

ok test 1 : start engine aand idle pressue comes up then falls alittle to 1750 cuts in runs til 2050-2100 psi cuts out. 7 pumps down til i hit 1350psi brake light turns on , countinue pumping down til gauge drops suddenly 16 additional pumps

What was the pressure on the gauge at the time it dropped like a stone.? That is (give or take) the accumulator precharge pressure. See Step 9 of Test #1.

From the way you say it is behaving it sounds to me as though the Accumulator Charging Valve is working OK.

 

[John C.]

Have you checked that there is no debris under the pedals or that the floor mat is not interfering? Do the pedals hang down from a mount above or are they mounted on the floor?

 

[mobile-tech]

nige , sorry per step 1 900psi and then drops like a stone , the pedals are floor pivot mounted i checked them out i can see the brake valve pluger and watch the movemnt looks to be bottoming out whever i work the pedal with my foot or hand , thanks

 

[Nige]

per step 1 900psi and then drops like a stone

So that should be approximately the acumulator precharge pressure, which is around 950psi depending on ambient temperature. Double confirmation that the accumulators do not appear to be the root cause of the problem.

I think that somehow you have to make sure that the pressure you measured in the system from the test point is actually reaching the brake valve. This might involve making some sort of temporary connection where one or other of the lines from the accumulators supply the brake valve. I can't see any reason why you should not be getting full system pressure there but it's worth a check.

Please ensure that nothing is messed up in the floor linkage (the left pedal is connected by a link to the right pedal). If it's all good then it seems as though you are going to have to go into the brake valve. See attached which is how it operates. Not the easiest thing in the world to get at unfortunately.
And I've saved the worst part for last - there are no service repair parts for it so it's a new replacement valve or nothing. It seems as though this is typical because I looked at a couple of other models that use a similar design of valve (but different Part Numbers) and they are not serviceable either. The Part Number for the valve illustrated below is 215-3687.

 

[mobile-tech]

yes i agreee im going to tee into service brake valve where accumulator pressure is coming in and verify i have correct pressure , i know i had a brake problem on our 815f brakes are simalar and the valve was serviceable but thats been a few years ago and that one the plunger would stick so it was a dragging issue , i will check it out and report back, thnaks brandon

 

[Nige]

Once you have verified the inlet pressure there is nothing to stop you tearing the brake valve apart for a look. Simply if you do find anything worn or damaged there are no spare parts available to repair it. At least not in Caterpillar’s system anyway.

 

[Nige]

i know i had a brake problem on our 815f brakes are simalar and the valve was serviceable but thats been a few years ago

The 815F Service Brake valve is a different Part Number 106-1774 and there is still a repair kit available for it. It looks to be significantly different to the one on the 938G.
257-7676 Kit-Repair (INCLUDES BOOT, SEALS, RETAINING, DETENT BALLS & SPRINGS)

 

[mobile-tech]

t into inlet of service brake valve 2000 psi at idle , step on brakes til 1750 psi cuts in runs up to 2100 psi cuts outs \
looks like the valve is next

 

[mobile-tech]

is there a way to check the percent left of brakes without diassembly ?
thnks

 

[mobile-tech]

im wrapping up for the day here , ill pull and disassemble valve if there are no crisis monday mournng and report back thanks

 

[Nige]

t into inlet of service brake valve 2000 psi at idle , step on brakes til 1750 psi cuts in runs up to 2100 psi cuts outs. looks like the valve is next

Here's a thought. Re-connect the inlet lines from both accumulators to the service brake valve.
Then try plugging the outlet side of both brake circuits at the brake valve, re-test and see how the pressure behaves. Then reconnect one axle circuit and re-test. Plug that one again, re-connect the other, and test again. Do you see any difference in behaviour between the front and rear brake circuits.?

is there a way to check the percent left of brakes without diassembly ?

Yes there is. See attached. The problem will be getting into the ECM to see the initial measurements when the machine was new. I assume you don't have ET.?

 

[mobile-tech]

ok so i finally got back on this ordered the service brake valve and installed it today , i now have 1250 psi and 1300 psi at outlet test ports of service valve ( theres my 50psi diff between fron to rear ) i havent disassembled old valve yet , i dont have cat E.T , im gonna install side pannels and take it for a run , thanks

 

[mobile-tech]

i did block off lines and test pressure it was stile low 300-400 psi , i belive that valve was bleeding to tank

 

[Nige]

i did block off lines and test pressure it was stile low 300-400 psi , i belive that valve was bleeding to tank

That's probably why you couldn't get any decent pressure to the brakes.

 

[mobile-tech]

pulled that valve apart today the spool valves where scored , machines stop excellent and has been back in service , thank you !!!!