cat 236 - i just rebuilt lift cylinders, but lift arms keeps leaking down

[makesumwake]

2003 Cat 236 skid steer, 3000 hrs

i rebuilt the bucket tilt cylinders last year. I didnt have to bleed them at all and the rebuild fixed the cylinder leak down issue

accordingly, i rebuilt the lift cylinders this year, hoping to fix issue of the lift arms settling down over time
but instead it didnt fix the issue, it got worse. im guessing i have air in the system, ive tried bleeding the lines by loosening them at the cylinder and collecting the oil that comes out, but that didnt help.

is there a special bleeding method to use? ive tried cycling the lift arms up and down 30 times probably trying to get air out


thanks
-phil

 

[Nige]

I think it is highly unlikely to be the cylinders causing a steady leak down of the lift arms. A small initial drop yes, but not a continuous fall.
See the diagram below. The oil below the piston is the full capacity of the cylinder barrel (diameter 2 - 2.56"), above the piston it is the barrel minus the diameter of the rod (3 - 1.77").
With hydraulic oil being basically an incompressible fluid if the cylinder moves down say 1" (with the control valve spool in the closed position) where does the extra oil go.? There isn't enough space for it above the piston because most of the cylinder barrel is occupied by the rod. If the piston moves down 1" it will displace 5.15 cu.in. of oil. In the same 1" of movement there will be only 2.80 cu.in. of space available above the piston for that oil to go.

 

[makesumwake]

right ive read this before, but i replaced the seals anyway just in case. one seal was broken completely inside so it needed it. i have an initial drop after lifting the arm, and then a steady slow sinking.
the initial drop wasnt there before the rebuild, so im guessing i have air stuck inside. and then the steady low leakdown i know is normal to some extent, but im hoping to be able to fix it.
how fast the arms sink depends on the weight in the bucket as well.

 

[Nige]

To exercise any entrained air out of the cylinders start by getting them in the mid-point of the stroke and with the engine at Low Idle and the oil at operating temperature. Start by moving the cylinders a couple of inches in each direction and back to mid-point again. Increase the distance you move the cylinders a small amount at a time over 4-5 steps but never to full stroke. Finally slowly take the cylinders from mid-point to full stroke in each direction and back to mid-point. If there is any air in the cylinders that ought to get it out.

I would think that the oil is getting back to tank via the (allegedly closed) control valve TBH.

 

[lantraxco]

Port reliefs and load checks in the control valve (if it has them) are the usual suspects. CAT liked to use something called "Resolvers" in some of their systems, damned nightmare as far as I could tell, but probably not on this small a machine?

 

[Coaldust]

Not saying this is the issue, just an observation from experience. Often, when I'm called to a customer to diagnose a hydraulic complaint where one thing has been recently fixed and now something else related is malfunctioning, I find a contamination issue. This is related to a dirty repair. Especially with hydraulic hoses.

Ports don't get capped, new hoses don't get cleaned before install, or dirt gets packed into unsealed components during install. (Side note) The average laborer, or operator that often gets tasked with a hyd. hose change doesn't have the knowledge of contamination control and can mess up a lot of stuff.

Next, the repair can go sideways when the hydraulic reservoir is filled or topped off. Dirty funnels, dirty buckets, dirty oil. Folks don't understand how clean it needs to be. The ISO 4406 Cat roll-off spec is 18/15 for your 236.

Like Iantraxco said, you will probably find a piece of crud inside a P/O check, or the port relief relief.

 

[Nige]

[QUOTE="Coaldust]Next, the repair can go sideways when the hydraulic reservoir is filled or topped off. Dirty funnels, dirty buckets, dirty oil. Folks don't understand how clean it needs to be. The ISO 4406 Cat roll-off spec is 18/15 for your 236.[/QUOTE]
The roll-off spec is now 16/13, the same as the fill oil spec. Those 789s I just put together were arriving with “Certified Clean Machine” tags with ISO Numbers in factory-filled systems as low as 10/8.......

 

[Coaldust]

Thanks Nige!
18/15 has been burnt into my brain. 10/8? Wow.

Everyone thinks I’m crazy when I mention new hydraulic oil needs filtered before going in the machine. And not with a rag or coffee filter. Lol

Ain’t nobody gots time fo dat. It’s new oil. It must be clean. It’s new oil. It’s in a shiny blue drum.

 

[Junkyard]

Pistons didn’t get put back on backward did they? Fixed a machine with similar issues and discovered that was the culprit. I’m inclined to it’s trash somewhere as well, I just wonder about cylinder assembly since it got worse after that change. I’ve never had any problems with purging air after a cylinder repair or hose change.

 

[Coaldust]

Often with leaking cylinders, folks wait too long to fix them. The piston wear band material chunks off, pistons scraping the barrel make metal, chunks of u-cup seals and buffer rings. Where does all that junk go?

The cylinders get replaced or repaired but there is now plenty of contaminants to mess up the works.

The contamination may not cause immediate issues. Or, the customer may think the new cylinders are the problem and point blame at the shop that repaired them.

Hydraulic repairs can escalate into crazy -expensive projects in a heartbeat.

Preventative MX is key.

 

[makesumwake]

Often with leaking cylinders, folks wait too long to fix them. The piston wear band material chunks off, pistons scraping the barrel make metal, chunks of u-cup seals and buffer rings. Where does all that junk go?
The cylinders get replaced or repaired but there is now plenty of contaminants to mess up the works.
The contamination may not cause immediate issues. Or, the customer may think the new cylinders are the problem and point blame at the shop that repaired them.
Hydraulic repairs can escalate into crazy -expensive projects in a heartbeat.
Preventative MX is key.

Coatdust, thanks for the comments!

ive had the Cat 236 for about 5 years. the lift arms had always sank down , i figured i needed cylinder rebuilds, but was just now getting around to do it finally since it didn't really effect overall operation that much.
i have rebuilt front tilt cylinders without issue, and it fixed the tilt cylinder leak down issues that i was having (or at least greatly reduced it to the point i didn't notice it anymore).

i rebuilt the lift cylinders the same way, being very careful to get everything back together the way i took it apart.
i did have a broken seal on one lift piston, not sure if there was chunks left over inside. i looked down the cylinder bore and didn't see anything, but of course it could be in the line or at a valve somewhere causing issues.

the overall leakdown rate of the lift arms is about the same as before the bebuild, but there is an initial quick drop after i stop raising the lift arms.
so with a bucket full of dirt, when i return the joystick from lift to neutral, the lift piston immediately sinks back into the cylinder about 3/4 of an inch over 4 seconds, and bucket drops accordingly.
and this DIDNT happen as much before i tried to do the rebuild. it happened a little but its way worse now.

then the sink rate slows down dramatically and the bucket sinks at the rate of 1in every 30 seconds (from measuring with a tape measure and stop watch). thats about the same as it was before i rebuilt the lift cylinder

 

[makesumwake]

also this is the first hydraulic machine ive owned, so i dont know what noises are normal.

1. for this machine, when the bucket tilt or lift pistons reach the end of their travel, and hit the stops, if the joystick isnt returned to neutral position, the engine starts chugging, like its overloaded.
i know if there is an overpressure situation in this case, there are internal relief valves that open fluid bypasses, but im not sure if it should be more smooth than this, or if a chugging engine sound is normal

2. during normal bucket movements, there is no noise from the hydraulic pump components, i just hear the engine running.
however when i steer left or right however (left side wheels going one direction, right side wheels going other direction), i hear a little bit of hydraulic pump whining noise. not sure if this is normal or not

thanks

 

[Nige]

The lugging down noise the engine makes when the cylinder pistons hit the stops is not normal. At that point the relief valve should open and you ought to hear the unmistakable noise of the hydraulics as the oil bypasses back to the tank.

Maybe the next step ought to be checking all the pressures - pilot, main relief, & line reliefs, as a minimum.

 

[Coaldust]

I agree with Nige. With the additional information provided, I don’t think the lift cylinders were the problem. It’s time to cut open hyd filters and check pressures.

 

[makesumwake]

Not saying this is the issue, just an observation from experience. Often, when I'm called to a customer to diagnose a hydraulic complaint where one thing has been recently fixed and now something else related is malfunctioning, I find a contamination issue. This is related to a dirty repair. Especially with hydraulic hoses.

Ports don't get capped, new hoses don't get cleaned before install, or dirt gets packed into unsealed components during install. (Side note) The average laborer, or operator that often gets tasked with a hyd. hose change doesn't have the knowledge of contamination control and can mess up a lot of stuff.

Next, the repair can go sideways when the hydraulic reservoir is filled or topped off. Dirty funnels, dirty buckets, dirty oil. Folks don't understand how clean it needs to be. The ISO 4406 Cat roll-off spec is 18/15 for your 236.

I did my best to keep it clean.
i cleaned off all the line connections before disconnecting them so that there wouldnt be excess dirt in the area once disconnected.
right after disconnecting lines i wrapped electrical tape around the ends of the fittings so keep dirt out
empty cylinders got capped with clean rags and tape
i made sure everything was clean and oiled before reinserting the pistons

 

[makesumwake]

I agree with Nige. With the additional information provided, I don’t think the lift cylinders were the problem. It’s time to cut open hyd filters and check pressures.

im new to hydraulics, so ill get a cat 236 specific repair manual and study up, do some tests and report back.

i imagine everything is serviceable without specialized expensive tools?
most likely i just need to replace seals or clean lines and valves out?

 

[Nige]

Post a Serial Number, or at least a S/N prefix, and I’ll look up the Publication reference of the Manual to suit your machine. Service Manuals aren’t cheap, I’ll warn you of that now.

Have you changed the hydraulic system filter recently.? If not I suggest that you do, and then do a filter cut on the old one looking for particles. Some good videos by a YouTuber called Adept Ape showing how to do it.

 

[makesumwake]

wow thanks for all the help!

serial number: CAT00236A4YZ04598

all filters changed last year at 3000 hrs
i follow adept ape also on youtube

 

[heymccall]

Is drift the same engine ON vs engine OFF?

 

[makesumwake]

Is drift the same engine ON vs engine OFF?

i havnt measured exactly yet, but from using the machine, im pretty the arms sink at the same rate regardless if engine is off or on.