Air in brakes

[CSC1540]

I have just replaced a rear brake cylinder , on the front I have replaced brake shoes and cylinders and one brake adjuster . the brakes have been adjusted without any issues .
When bleeding the brakes and after pumping, the brakes feel firm and all seems well but if I take my foot off the brake pedal for a few seconds the pedal will then go to the floor, so I assume air is getting in some how . The nipples are tight , I have traced the brake pipes looking for leeks , there isn't any brake fluid at the bottom of the drums. I haven't tightened the hand brake yet but that should be ok. Is there a way of finding out exactly where the air is getting in ?
I have bled the brakes about six times now and have ran out of ideas .

Regards

[cmwbob]

for the pedal to go to the floor after seeming to be good would require a highly visible loss of fluid, so its almost certainly not a leak or air entrainment. I would suspect the master cylinder letting by, sticky valve or debris preventing seal.
Full strip down of master cylinder and ideally a service kit for it. well worth the effort even if it only eliminates it as a source of the problem. If the slave cylinders needed doing then the master cylinder is a logical conclusion to get the same treatment.

[notsmartcar]

I’m guessing you have drums alround ..adjust brakes tightly so the drums won’t rotate and see if you have the same problem

[CSC1540]

Thanks both. I will try the easy one first

Regards

[hutchii]

Hi
By the sound of it you've been both unlucky and lucky at the same time! Unlucky that the master cylinder has (I think) failed, and lucky that it didn't happen on the road!
If you're satisfied that you've expelled any air from the system, If there's a leak anywhere, maintaining pressure on the brake pedal should result in it "creeping"
Personally I wouldn't repair the master cylinder but replace it. Yes it's more expensive but what price safety? A repair kit will get you new seals but an old bore, and if that's not perfect, you're back to square one.

[CSC1540]

Thanks for that . I wont be repairing the master cylinder, I don't have enough confidence in my own ability and although I haven't done the job before , doing a replacement might be easier . I am just off to double check the bottom of the brake drums for any fluid.

Regards

[administrator]

Are you seeing air in the lines when you bleed them?

If not, you may be experiencing one of the duff cylinder boots. These are so inflexible, that they pull the pistons back in after you apply the brake, making the next press less-than effective. Usually, you can spot this by pressing the pedal a second time and it will firm up.

Fix is to re-use your cylinder boots from your old cylinders or place a spring between the two pistons in the cylinder to prevent them creeping together.

[CSC1540]

Hi Admin.. Thanks for the response , there are no bubbles coming through. I can pump the pedal ten times and it seems fine. once I stop for ten seconds, I can then push the pedal all the way to the floor.

Regards

[notsmartcar]

Some times the new shoes radius is different to the drum so when the pivot is correctly adjusted the shoe in use will touch half way on its length and cause springyness at the piston end ...don’t forget drums alround NOT LHM fluid.

[SILLYCVS]

Brake adjustment is my thought, or it wasnt unknown for pistons in wheel cylinders to recede on own causing your symtoms