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Cat 950F Hydraulic Brake noise

Liviu Popescu

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Aug 19, 2020
Messages
52
Location
Romania
Hi all. We have some issues with our wheel loader, CAT 950F Series II. From what I can tell they are hydraulic over hydraulic brakes. Serial number is 8TK01037.

There is a strange noise coming from around the accumulator charge valve. It started happening immediately after we replaced the orange hose (was originally a steel line), it connects the accumulator charging valve to the pressure reducing valve. Back then it only happened once a month and would go away when you pressed the brakes.
Then we replaced the pink/blue line with a hose which connects the pressure reducing valve to a manifold which I think connects to the brakes.

Our operator seems to think it’s air but from what I’ve checked on the SIS there is no way to remove the air as it runs of the main hydraulic pump so should be sorting it out itself.

Before the pink line replacement, we changed the main valve group(as it cracked) and then hydraulic oil but haven’t worked with the machine since.. just started this week.

Brakes work ok but we’re worried for the noise. Noise goes away briefly when you press the brakes.
Unfortunately, we have no pressure test equipment as never had any issues with these machines.
If anybody has any ideas/suggestions it would be of great help.

I’ve uploaded a short clip on youtube maybe someone recognizes the sound (please let me know if it doesn’t work).

https://youtube.com/shorts/wK-5ZLEa9Xo?feature=share

D4234428-D1BA-4772-AF83-5F0938FDFE25.jpeg D33AB7B4-79BA-4A83-B909-078B10B18DBE.jpeg 59C8FB58-FCD6-4F98-B0BC-D13C88DCA24E.jpeg

Many thanks,
Liviu
 

Nige

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Have you any way of checking the charge pressure in the accumulators.?
That ought to be the first place to start, especially if they haven't been checked before.
Also I don't think replacing steel lines with flexble hoses (other than for a temporary repair) is a good idea on a brake system.
 

Liviu Popescu

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Joined
Aug 19, 2020
Messages
52
Location
Romania
Hi Nige, thanks for the reply.

We don’t have a test kit for the pressures, I always said I’m going to buy one but got a bit confused over what gauges and quick couplers I need - I suppose you check the accumulators pressure by plugging in a gauge to the accumulator charge valve.

I always thought of buying a Chinese Sinocmp test kit but always hesitated as didn’t really know much about them and then CAT ones are so rare on ebay and when they come on board they cost a lot.

So accumulator charge would be my first point to check, thank you.

You’re right about the brake lines, very good point!
 

Nige

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I suppose you check the accumulators pressure by plugging in a gauge to the accumulator charge valve.
If you look under the 2-bolt cover on top of each accumulator you will find what looks like (but actually isn't) a Schrader tyre valve. The procedure to test the accumulators is in SIS which I understand you have access to. To be honest I would investigate the possibility of borrowing or renting the test kit to do the accumulator precharge pressure check. It's not like you will use it often.

upload_2022-3-21_17-2-43.png
 

Liviu Popescu

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Aug 19, 2020
Messages
52
Location
Romania
Thank you, I’ll investigate if we can get hold of this and come back to you. I know there is a place where we charged the nitrogen accumulator on the hydraulic hammer so am thinking if it’s easier to just take the accumulators to them.
I’ll see if there is anything to watch put when removing these.

thanks
Liviu
 

Nige

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I know there is a place where we charged the nitrogen accumulator on the hydraulic hammer so am thinking if it’s easier to just take the accumulators to them.
That might be a good idea. The accumulators are not difficult to remove.
I’ll see if there is anything to watch out when removing these.
All you need to do before disconnecting anything is make sure you have removed all the pressure in the brake system. To do this (with the engine stopped), press the brake pedal to the floor anything up to 30 times until it goes "hard" under your foot. At that point there is no pressure in the hydraulic side of the system.
 

Liviu Popescu

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Joined
Aug 19, 2020
Messages
52
Location
Romania
Have you any way of checking the charge pressure in the accumulators.?
That ought to be the first place to start, especially if they haven't been checked before.

Just for my own benefit and that I can learn some more, what do you think is making the sound if it is the accumulators?
 

Nige

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If the accumulators are very low on charge they will be cycling almost non-stop until you press the brake pedal at which point all the flow from the brake pump goes to the brakes.
I'm not saying it definitely is the accumulators, simply trying to go through the diagnostic process step by step until the culprit is revealed.
 

Liviu Popescu

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Aug 19, 2020
Messages
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Location
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Hi all, small update - we tested the accumulators as per the manual (buildup pressure, turn off ignition, and press brake pedals to see when alert comes on). The manual says it should come on after at least five pumps/presses, ours came on after the first 1. We’ll have them regassed next week I think while they’re blasting the quarry. Will keep everybody posted.
 

Nige

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The manual says it should come on after at least five pumps/presses, ours came on after the first 1
And I would suspect that the standard of "at least five pumps" of the brake pedal ought to be nearer to 10 if both accumulators are in tip-top condition.

Before servicing the accumulators please remember my suggestion to check for any oil in the gas side of the accumulator that would indicate you need to re-seal them before re-gassing. It can be done quite quickly as follows: -
1. Remove the protective cover and the dust cap of the accumulator charging valve.
2. Install only a gas charging chuck (without hose) to the connection on the charging valve. Tighten the chuck on to the stem of the charging valve.
3. Loosen the locknut on the charging valve.
4. Make sure that the hose connection side of the charging valve is facing away from you.
5. Gently screw down the T-handle of the chuck to very slightly open the charging valve. As gas starts to flow from the accumulator carefully observe the valve for any spits of oil coming from it. The presence of any oil at all indicates the accumulator needs to be fully discharged, disassembled, and re-sealed before nitrogen charging is done.

upload_2022-3-23_16-38-23.png
 
Last edited:

Liviu Popescu

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Hi all,

Update: both accumulators are now charged. We were lucky that our guys who do the drilling for the blasting offered to refill our accumulators.

The downside is that, although I told everybody to check for oil as Nige suggested, everybody forgot. I have understood though that they were not completey empty which might mean the seals are ok, but I guess we will see soon.

We’ve stopped for the blast so haven’t worked with the loader but initial feedback seems that the noise is gone. We will see next week when we start again. Nige was spot on with this advice - after the accumulators were charged it now takes 10 pedals for theoressure light to come on.

I’ll keep everybody up to date and thanks again for your help Nige!
 

Nige

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Just to be clear I meant to look for the presence of oil at the nitrogen charging valve end of the accumulator - which should be dry if the seals are in perfect condtion.
Worn seals you might observe a few spits of oil coming out when the gas pressure is released. If the seals have failed then significant quantities of oil will be present in the top end of the accumulator.
What you have to always bear in mind is that if the seals are even slightly worn then the nitrogen pressure in the accumulators will reduce over time as the gas leaks past the seals and into the oil side of the system.
 

Liviu Popescu

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Messages
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Location
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Hi Nige, we now have developed a new issue.
The operator was working today and all of a sudden the parking and low oil pressure brake light came on. Stopped to check and everything seemed ok and worked with it a little more. However after that, the loader braked by itself and is now “braked”.
They can see the piston from the parking/secondary brake slowly move back but as soon as you apply brakes, switch it off etc, it becomes stuck again.

It also doesn’t want to go into gear easily (unless you put it in reverse and the fwd) but I think that’s to do with the parking brake switch remaining closed.

I’m not sure where to start with this, from what I have read, the oil to the secondary / parking brake valve comes from the front accumulator and they think there is low pressure going to this valve. The lights for parking brake and low brake pressure are also on.

They removed the parking valve to see if it’s clogged but all appeared good.

I have 2 theories: either the rod/bellcrank from the parking valve is stuck/catching and that limits the flow to the valve or something has now happened with regards to the accumulator side and the charging valve.

When we charged the accumualtors, we did not touch any of the valves so it worked before with the accumulators drained and also for a few days with them charged.

any feedback would be appreciated as we just blasted last week and were ready to finally start the season.

many thanks
Liviu
 

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Nige

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First suggestion would be to first put a pressure gauge on the air system test point, start the engine and see what happens to the pressure. It should go to around 120psi before the compressor cuts out, and cut back in again at around 95psi.
Second would be to put a pressure gauge at the test point for the hydraulic side of the brake system. With the engine running it should cycle in the approximate range 1700-2100psi.
If you have the low brake pressure alarm sounding then unless the brake system pressure sensor has failed you should see a pressure far below "normal" on the gauge.
 

Liviu Popescu

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Hi Nige - it’s hydraulic over hydraulic brakes, there is no air compressor present. 8TK01037 is the S/N.
 

Nige

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In that case go straight to the hydraulic pressure test. The pressure should still cycle between 1700 and 2100psi, controlled by the accumulator charging valve.
Sorry, I thought it was air-over-hydraulic. Should've checked first. (It seems as though earlier models are air-over-hydraulic and it was changed to full hydraulic during production)
 

Liviu Popescu

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Hi Nige - would you know roughly where that is? If not I’ll dig in the system and am sure will find it.

Thanks
Liviu
 
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