Pedal sinks to floor if left 5 min then goes back to normal

[rcx106]

I took my car off the road for an overhaul and now I'm having this strange symptom since putting it back together. If I pump the pedal a few times it goes nice and firm. If I leave it a few minutes then next time I press the brake pedal it goes near to the floor again. A pump or so later and it's back to firm.

During the overhaul I did these things:

Replaced the two front calipers with re-manufactured ones
Replaced brake hoses with new ones
Replaced one of the brake pipes for a copper/nickel one
Replaced the load sensitive proportioning valve with a used one from a car I broke up (was working when removed two years ago)
Drained the master cylinder so that I didn't have fluid dripping while the calipers where off

I've bled the brake using a vacuum pump in the order mentioned in the factory manual. The manual does however say to bleed the master cylinder by letting air out at the master cylinder outputs. I haven't done that as I don't have anyone to help with this. I thought maybe if I'm using a vacuum pump then maybe this isn't needed.

Problem wasn't present when car was taken off the road. The rear drums haven't been touched. Brake servo wasn't touched. I've checked for leaks but cannot see any.

[Dave Brand]

You've got air in the system. Pumping the pedal compresses the air, which expands again when pressure is released.

Why not just do the job according to the manual? These things are there for a reason!

[rcx106]

Hi Dave, agreed. My reason for asking is that (a) I don't have anyone here to help and (b) I thought that using a vacuum pump will automatically also bleed the master cylinder.

Anyways, I've convinced my girlfriend to come round and spend a couple of hours in the cold garage helping out with this!

[rcx106]

Dave, when brake fluid gets transferred from the 5 gallon container it came in, into a smaller container then to the reservoir, by that time the fluid has accumulated some bubbles. I'm guessing these bubbles make things worse so I should look for a method to avoid this. I remember being taught how to bleed brakes on high end mountain bikes and the fluid was put into a syringe, the syringe closed and the piston pulled out, creating a vacuum which sucks the air out of the fluid. That's probably taking it too much and maybe pointless on a car seeing as the reservoir on a car is vented anyway unlike the sealed container on a mountain bike.