David Brown 990a Hydraulic / bypass valve issues please help

Ok, So basic problem is 3 point hitch does not do anything, no lifting, no lowering.

so far:
Dropped transmission pan
New filter
cleaned screen
cleaned magnet
poured a small amount of diesel through while filter was off
sprayed carb cleaner into area.
replaced filter on top left side of rear end
added new fluid
cleaned control valve, removed both side covers

Filter was milky, clumpy, discolored, particled, plugged. looked aweful.

3 point hitch still does not work. Previous owner told me it needed a "new" by-pass valve. I am new to working on tractors but very mechanically inclined. Also before posting ive been researing this issue on this forum, the david brown UK forum and several others. At first i had no clue where the bypass valve was, but i am certain i have found it. The right hand side there is a needle, the left hand side i cannot tell if it is empty (missing valve) or if it is stuck in there. I took a wooden dowel, shaved the head of it to look like a pencil, hammered it in and twisted, sprayed with wd40, pb blaster, let it soak, tried again and no luck getting the piece thats their out. I have taken several photos, i took a photo of what the dowel i made looked like after being hammered into the shaft. From the diagrams i have looked at, there should have been a spring and a ball bearing which was not there. Please help!!

thanks in advance!!!

the last picture is of the right rear cover, the left spring is missing something, i saw in a diagram they just call it a pin, what does this thing do, do i need it ?

thanks.

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The valve still appears to be in the bore. Did you remove a cap and spring from it?

On very stuck ones, I have had success using a a 7/16-20" tap(Lock a pair of vise grips on it) to get enough bite on it to get it freed and out.

They will still work when the guts of the valves are removed(ball bearing, screen etc.)
 
(quoted from post at 11:53:49 03/03/15) The valve still appears to be in the bore. Did you remove a cap and spring from it?

On very stuck ones, I have had success using a a 7/16-20" tap(Lock a pair of vise grips on it) to get enough bite on it to get it freed and out.

They will still work when the guts of the valves are removed(ball bearing, screen etc.)

Yes, there was a round screw cap for a big flat head just like the other side. I used a bolt extractor and got the valve out, it was def stuck. I polished it up and cleaned it. I started the tractor with it out and theres no fluid coming out of that side. The other side with the needle has fluid coming out, i backed the cover out and let it bleed, it was solid fluid no bubbles. I reinstalled the by-pass valve and its no longer stuck, i can spin it with my finger. But still no 3 point hitch.
 

There is one more issue with the control valve, there is a broken off lever. The valve it actuates i think is for auxillary hydraulics, but i am not sure. When i had control valve off, it was frozen and i lubed it and got it freed. but i dont know which way it is suppossed to be for 3 point hitch to work ie parrallel or horizontal, in or out. I know if i pull valve all the way out the screw cap in the 2nd picture to the right fluid will come out of there. thanks.

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(quoted from post at 15:46:05 03/03/15)
The valve it actuates i think is for auxillary hydraulics

Yes, for remote hydraulics. Center position will allow flow through.



The 3 way pointer over the right rear cover... Is it in the center position?

The piece you are missing from inside that cover, is a push rod that sends input to the valve body from a cam on the end of the rockshaft. It is a couple pieces of steel plate, with a roller sandwiched between them. The roller rides on the rockshaft cam, the short leg rests on the spring, the long leg appears to still be in there, but broken off(the bar that slides up an down, not attached to anything with a hole in its top).

Use a pry bar to push down on the lever that is just below the bottom left bolt head under that cover. That should allow the spool valve to open, and give you lift if everything is OK on the hydraulic side of the system. If it works then I can help you with the broken/missing mechanical stuff.
 
(quoted from post at 14:36:44 03/03/15)
(quoted from post at 15:46:05 03/03/15)
The valve it actuates i think is for auxillary hydraulics

Yes, for remote hydraulics. Center position will allow flow through.



The 3 way pointer over the right rear cover... Is it in the center position?

The piece you are missing from inside that cover, is a push rod that sends input to the valve body from a cam on the end of the rockshaft. It is a couple pieces of steel plate, with a roller sandwiched between them. The roller rides on the rockshaft cam, the short leg rests on the spring, the long leg appears to still be in there, but broken off(the bar that slides up an down, not attached to anything with a hole in its top).

Use a pry bar to push down on the lever that is just below the bottom left bolt head under that cover. That should allow the spool valve to open, and give you lift if everything is OK on the hydraulic side of the system. If it works then I can help you with the broken/missing mechanical stuff.

Gave it a shot and unfortunately it still does not lift. In that same area with the cover off and the tractor running there is a lot of fluid circulating, when you raise the control lever, or pry that little lever down, the pressure goes up. I attached a picture. You can see the fluid on the right, maybe theres a leak ? not sure if this is normal.

thanks.

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If the oil didn't stop, that mean the bypass valve isn't seated(again, this assumes everything else is good).

Are you sure it slides all the way up and down in its bore freely? It needs to glide with no resistance. Polish the valve with some 600 grit paper till it shines(try not to round over the sharp edges), and install your dowel rod. Work it up and down a few 100 times in the valve body.


Can you feel the bypass come to a hard stop at the bottom?
 
Where are you? Nice to see some grass on the ground. It has been months since I have seen any here.
 
(quoted from post at 18:05:29 03/03/15)
I am in central Florida, I will work on the valves in the morning hoping I get it going thanks for all the help!!

Ok, so re-polished the by-pass valve, ran it up and down a couple hundred times, it is smooth now, moves very freely. Still no 3 point and the flow hasnt changed in the right rear cover. Pulled the cap back off with the tractor running, manually moved the by-pass valve up and down with wood dowel no change. There is no fluid at all in the by pass hole. It doesnt seem like the by-pass valve is going all the way into the seat, but i cannot tell. It seems like it only goes down to the edge of the threads that the cap screws into, i would think it should be able to move up and down at least .25" and there just doesnt seem to be clearance there. There is pleny of fluid on the right hole with the needle in it, i pulled it back out and took a pic theres a washer down on the hole and i held it the way it came out to verify its in there properly, i tried flipping it over but the mushroom head is too big for the washer. I assume previous owner was in here before so something could be installed wrong. Also is there any difference in the 2 caps ? They both look identical, it seems they are the same height and same inner diameter. thanks.

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I've been all over the case parts site for schematics, this is the closest one i have found but it is quite a bit different. It says the valve i have been fooling with is the "hold valve". That hold valve set up is way different, looks to have a cylinder that screws in the the hold valve can slide into. The one next to it is the bypass in this schematic, but mine doesnt have that, it just has a needle next to it. So i guess im dealing with a hold valve. is there a by-pass valve somewhere else on this machine ?

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more pics for reference. I also still dont know what exactly center is on the auxillary lever, i can spin it 360 degrees, so i need to know if the shaft should be horizontal or parallel.



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****UPDATE*****

Some progress made, the lift goes up now, but not down. So underneath the needle is the bypass valve. I did the wood dowel trick, polished it up with 1000 wetsand paper. worked it in the hole several hundred times. Then still nothing. So i decided to pull the needle out and see if maybe it went on the hold valve, but it was sticking out to high for screw cap to tighten. So i decided to run it without the needle. I bled system, checked for fluid on LH front side valve in front of rear end and i had fluid. I let it bleed for a few but still nothing. I figured if i had fluid there the lift would have to work, so i revved up the engine and presto, the lift arms raised. But when i tried to lower nothing happened. So back to the right side but this time the cover on the front of the rear end. I had the adjustment nut almost all the way down to give more pressure, i backed it out til just one thread showing, lowered control lever but arms still wont go down. There are no springs on either side, none in the by-pass, none in the hold valve. I have a spring assorment, do i need to put one in one of these ? What do i do with the needle ?

thanks.
 

Worked on it some more, re-polished the by-pass valve again, tried it with needle in 2 tries nothing, third time went all the way up. I checked the lever on left rear axle the lock but that isnt the problem, as soon as i crack the nut holding the hold valve in it releases the pressure and the lift goes down. spinning my wheels now, will go onto something else, still need pointed in the right direction !!!

thanks!
 
I have never seen that needle before in one. Maybe it is a Homemade part, trying to "fix" some other problem with the system.

With the arms up, try playing with the lever I told you to pry on earlier. It could just be the mechanical side of thing are giving you the "up all the time" issue.


The DBTC has a basic fault finding guide with some good schematics that show the valve body with names, and a flow schematic.

http://www.dbtc.co.uk/index.php?module=Content&func=view&pid=182
 
I would try to help you but I'm so dumb that I'm still trying to figure out the hydraulic systems on my 990s (after about 7 years of ownership). One of mine has woes similar to yours. I see you posted on the dbtc website. On that website is the following link, which you probably have already seen, but in case you have not, here it is. It may help. It is FAQ #5 on that site (if the link does not work).

http://www.dbtc.co.uk/index.php?module=Content&func=view&pid=182

I am getting a lot of useful info off your thread. Please be sure to post the final fix. Thanks
 
(quoted from post at 17:54:27 03/04/15) I would try to help you but I'm so dumb that I'm still trying to figure out the hydraulic systems on my 990s (after about 7 years of ownership). One of mine has woes similar to yours. I see you posted on the dbtc website. On that website is the following link, which you probably have already seen, but in case you have not, here it is. It may help. It is FAQ #5 on that site (if the link does not work).

http://www.dbtc.co.uk/index.php?module=Content&func=view&pid=182

I am getting a lot of useful info off your thread. Please be sure to post the final fix. Thanks

Like i said, i am new to tractors, i would love a "david brown guide for dummies" Or something with laman's terms.

Quadrant lever - What is a quadrant lever

is this the lever with the broken lever on mine or is it the lift control lever ?

I've seen alot of things referencing the TCU valve sticking, i am seeing that this is underneath the control valve, so i am thinking the machine has to be torn down and control valve removed to access ?

My other main concern at this point is the hold valve, It does not seem to have any room to move, if i loosen and/or remove the cap for the hold valve, the fluid comes right out, its pressurized and the arms go right down.

there is no spring on the hold valve, or bypass valve, no guts and no room in the hole for the bypass valve to move.

I am still thinking these are issues, maybe not so much with the bypass but with the hold valve.


I hate to rip apart the machine without solving these issues.

I have not had time to fool with the tractor in the last couple of days, i also did not check for a plugged tip on the hold and by-pass valves. With the right rear cover off, the oil flow never seems to stop. I will mess with it some more, but i would love to get some more input before i remove control valve and check spool valve and abutment plate.

thanks!!!
 

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