Just fitted a brand new toilet with a grohe air flush and it will flush only if the button is held in

Grohe Flush Valve Replacement

hey, since it is new, I assume a small stone or left-over is stuck where the seal is, You need to disassemble the valve, clean it and re assemble. Look at this guide that might help you. though it is not the exact model, principle is the same.
Have fun
Avi Grohe GD2 flush system concealed cistern repair fix and disassembly...

The following procedure is extracted from the following link:
How do you replace GROHE single flush valve part no 43544 My discharge...

1 Lift off Cistern lid
2 Remove the bridge carrying the end of the tube from the flush button and the bellows below it. There are small catches at front and back.
3 Take hold of the tube and lift, the whole of the rest of the works comes out.
4 The lowest part has two washers and is held to the upper by one of them it can be removed from the upper by pulling it. then examine the two washers. There may be "bubbles" on either or both. These are the cause of the loss of seal and can be removed by pricking them with a pin but DO NOT pierce the washer.
5 Reassemble

hey,
If it is new, I assume a small stone or left-over is stuck where the seal is, You need to disassemble the valve, clean it and re assemble.
If it is an old installation, it is possible that the seal is worn and need to be replaced.
Look at this guide that might help you. though it is not the exact model, principle is the same.
Have fun
Avi

Grohe GD2 flush system concealed cistern repair fix and disassembly...

Switch Off Supply Remove Pipe Going To Unit, Inside You Will Find Small Filter ,Clean Filter Replace And Re Fit Pipe .
Turn On Supply Check For Leaks, Rectify If Necessary. Problem Will Be Solved

hey,
If you can get a new bellows this is possible.Repairing a bellows is also possible if you have the skills and tools to hot-air solder it.
Look at this guide that might help you. though it is not the exact model, principle is the same.
Have fun
Avi

Grohe GD2 flush system concealed cistern repair fix and disassembly...

hello Bill, the other end is attached to the valve pneumatic unit.

Look at this guide that might help you. though it is not the exact model, principle is the same.
Have fun
Avi

Grohe GD2 flush system concealed cistern repair fix and disassembly...

That is a tricky one. Tighten the tabs, twist into place, then tighten a bit more. They are a pain in the a$$, but with patience you will get it done. Good luck

hey,
Check your button/s panel, check that the hoses are connected firmly.
Disconnect the hose and blow air through it, does it work now?
If it works ok, you need a new buttons panel. (or fix it).
If problem remains, you need to disassemble the pneumatic valve and check / clean/ repair it.
Look at this guide that might help you. though it is not the exact model, principle is the same.
Have fun
Avi

Grohe GD2 flush system concealed cistern repair fix and disassembly...