brake pedal goes to floor when I turn on the car

[raltz]

hello all, i've been searching and can't find any solutions...

i am not familiar with working on brakes, but i disconnected one of my break lines and when re-installing it i was told i had to bleed the brakes, so i pumped the pedal while my dad was under the car opening the bleeder and closing, starting at the rear passenger side to the driver back to the rear driver to the front passenger...

the pedal started feeling tighter and how it normally did...but when i start the car the pedal goes loose and im able to easily slam it to the floor...while its on if i keep pumping the brake pedal i feel it getting tighter, but as soon as i stop and push it again, it goes back to being loose...

any ideas as to what im doing wrong, or if something is dead and needs to be replaced?

done on:

1973 datsun 240z, original factory drum and disc brakes...

thank you

[Reverend]

There is still air on the system. I had similar issue and by replacing the brake master cylinder it went away. So was your bleeding order this (sorry, not native English speaker so i need to clear it up): from rear passenger to front driver then from rear driver to front passenger? If you did that, it went wrong. It should go: rear passenger->rear driver->front passenger->front driver. BEFORE you do all this, remember to bleed it also from the master cylinder.

[mlc240z]

Agree, you still have air in the system.

On a Z the longest brake line run is the driver's rear, different from most cars.

So start to bleed in order, driver's rear > pass rear > pass front > driver front. Pump brakes slowly, full strokes and then hold.

Have someone open bleeder while maintaining pressure until pedal goes to floor. Then shut bleeder and repeat until you no longer get any air bubbles released. Make sure to have a tube going from bleeder to a container of brake fluid so you're not sucking air back in to the system when you release the pedal.

[raltz]

Thank you guys, but just wondering does the car have to be on when i bleed it? Because when the car is off and i do a round of bleeding, the pedal feels fine, but as soon as i turn the car on, thats when the pedal goes to the floor

[FastWoman]

No, don't run the engine when you're bleeding the brakes. The sinking of the brake pedal when you start the engine is the result of vacuum being applied to the brake booster, which pushes harder on the master cylinder's piston. However, pedal movement is all that is important. Moreover, you don't want to bleed brakes with the sort of pressure that will be created with the master vac working.

You say the brakes will pump up, but will fade away if you keep steady pressure on the brake pedal. This makes me think you also have a bad master cylinder. If the car has sat a very long time (years), then I would not be surprised if you have multiple problems with the brake hydraulics (air in lines, bad master, bad cylinders, bad calipers).

[Arne]

Might have over-stroked the master cylinder when bleeding. That's a common problem. If the master cylinder is old, pushing the pedal too far during bleeding can run the piston seals over sludge in the bore and damage the seals. If bleeding properly doesn't bring it back, you may need to replace the master cylinder.

[raltz]

Thank you again guys, so what would you guys recommend to do to determine if the master cylinder is really the problem

[Reverend]

It is hard to say (for me at least) i just replaced it because bleeding didnt solve the problem. After new master, everything went better than expected

[raltz]

Thanks guys for your help! New master cylinder it is