M20C flap pump...

[AviatoRob]

We installed a LASAR overhauled pump and bled it. Comes down fully and firmly in 4 to 5 strokes. The flaps will not remain down however. After a few seconds they retract gradually to the full up position.
What's the deal? 

[Rwsavory]

Here is a troubleshooting guide.

66' Money M20E Flap Pump Diagram.pdf

[bixmooney]

I had a couple of issues with this, kind of finicky. For me, it was a slipped push pull cable, and I can attach a picture of that and another diagram that has been posted previously of how the valve works.

[M20F]

If HE-1517 are worn out the situation you describe occurs.  I finally managed to source a replacement pump to fix my bad seals. 
 
Also note there are two springs.  One is strong and one is weak.  If you take apart note which is which, I forgot which of the two it is but it caused me a lot of grief till I understood the springs aren’t the same and need to go in the correct way.  
 
@Shadrach has a lot of insights on flap operation. 

[Skates97]

And it's very helpful to have it sitting on a towel when you take it apart to keep that little steel ball in between the two halves from running away....

[Immelman]

Before doing anything with the pump, I would look closely at the rigging of the cable that connects to the retraction lever in the cockpit to the small pivoting arm on the side of the flap pump. On my E, the cable is clamped to the pump lever with a screw. As I recall it was tricky to get the positioning correct on this. Will the flaps stay down if you disconnect the cable, and have someone manually position the arm on the pump to various places along its travel? This would help isolate rigging vs pump. By all means do this before taking the pump apart and having to re-bleed the system.

Edited August 7 by Immelman

[BillyT0020]

Id also be suspect of the Lasar “overhaul”. I know at-least 2 people that  got bad pumps from them recently. 
 

Billy 

[Shadrach]

If the flaps bleed up but the flap handle remains bottomed out, then the leak is likely in the return valve.  
This could be an improperly adjusted return cable or a leak in the valve itself.

if you’re unable to resolve the issue by adjusting the return cable, then the next step is to investigate the valve.

One way to verify that the leak is in the return valve is to tighten the return speed adjustment screw clockwise until it bottoms out. This will block the return orifice.  If your flaps stay down after doing this, then you know that the return valve is the leak point. The only way to correct it is to remove the pump and recoin the valve to ensure that it seats.

[Shadrach]

On 8/7/2025 at 1:18 AM, BillyT0020 said:

Id also be suspect of the Lasar “overhaul”. I know at-least 2 people that  got bad pumps from them recently. 
 

Billy 

I hope they’ve gotten it together… Last year I talk to two different people who were chasing their tails, trying to set up hydraulic flaps. It turned out to be pumps that were improperly assembled during overhaul.