Intermittent Brake Failure.

[Yorick]

The missus has a little AJP125 and I recently fitted new front caliper and master cylinder.
She's had a few rides and all OK. This morning I rode if from garage to drive and noticed how solid the brake felt.

But this morning the brake just went spongy and failed approaching a junction. She was going slow anyway so no problem.
She stopped, then pumped the lever 3 or 4 times and it came back.
She tested the brake many times on way home and no problems.

I've never known a brake to just fail, then come back.
I've been prattling with brakes for nigh on 50 years so quite experienced.

Any ideas or suggestions?

[dern]

Take the pads out and look for odd wear patterns that suggest that they're not seated correctly or the pressure is being applied unevenly.

[Le_Fromage_Grande]

The brake probably needs bleeding, but could be master cylinder seals - basically take it to a bike shop and pay for them to fix it.

[Yorick]

The brake probably needs bleeding, but could be master cylinder seals - basically take it to a bike shop and pay for them to fix it.

Everything is brand new. So shouldn't be problems.

[Le_Fromage_Grande]

Then it needs bleeding, you could try a cable tie holding the lever in for a week, sometimes this fixes it.

[DefTrap]

As you're not frit of braking systems, I'd just take it to bits again, inspect it all and refit.

[KungFooBob]

Sounds like something pushed the pistons in. I can't imagine it was a tank slapper, I'm sure she would have told you about it.

Have you ridden it, could the disk/calliper be loose, or is the disk warped?

[mangocrazy]

What was wrong with the old master cylinder and caliper?

[mel]

Then it needs bleeding, you could try a cable tie holding the lever in for a week, sometimes this fixes it.

This ^, although overnight should do it if indeed there is a small amount of air in it

[roadster]

Make sure that there is a gap between the master cylinder piston and the lever when the lever is released and also that the piston is returning properly so that the port connecting to the reservoir is open. Might be a problem if the lever travel has been mechanically reduced to suit smaller hands for example.

[Yorick]

I took the caliper apart and one end of one pad wasn't held in situ. Wasn't gonna fall out, but didn't seem right.
I still have the old caliper and that had a tube/spacer thingy.
That's on the new one now. Fingers crossed.

I'm a bit confused coz I bought a new caliper and expected it to be complete.

And bled it fully. Seems OK.

Thanks for all input.

[A_morti]

Everything is brand new. So shouldn't be problems.

Especially because it's a Chinese bike, but in general too, I wouldn't always assume new equals good.