Brake pedal sinks to floor engine on

[Cdog_96]

I am stumped, I have replaced my brakes all the way around, new MC and brake booster, bled the lines several times and my brake pedal goes all the way to the floor with the engine on. With the engine off the pedal feels good and doesn't sink, with the vacuum booster plugged off pedal feels good also. It seems the booster is working due to it over assisting the pedal with the engine on. Master cylinder is a new ac Delco. No fluid leaks anywhere, I don't really know where to start and I don't wanna replace new parts unless I have to . Any help appreciated thank you

 

[squaredeal91]

I know you said new master cylinder but it sounds just like a bad mater. Did you bench bleed it good before installing?

 

[Ricko1966]

Plug all the brake ports at the master,if the problem goes away replace just your front lines. Did the problem stay away? Replace your rear lines,did the problem come back? Now we've narrowed out a lot. Use this strategy in which ever combination you need too,to narrow down the problem.

 

[Cdog_96]

I know you said new master cylinder but it sounds just like a bad mater. Did you bench bleed it good before installing?

Yes I spent a good hour on it, but it has been a while so maybe a possibility

 

[Cdog_96]

Plug all the brake ports at the master,if the problem goes away replace just your front lines. Did the problem stay away? Replace your rear lines,did the problem come back? Now we've narrowed out a lot. Use this strategy in which ever combination you need too,to narrow down the problem.

What do you mean plug the brake ports ? You mean where the brake lines go in ? How will I do any brake operation I think I am misunderstanding

 

[Ricko1966]

Where the brake lines screw into the master get plugs and plug it does your pedal still sink. Ta Da the problem is at the master. Things are fine hook up the fronts,does the problem come back? Ta Da the problems in the front No ? Hook up the rears,Does the problem come back? Ta Da the problems in the rear.

 

[Cdog_96]

Where the brake lines screw into the master get plus and plug it does your pedal still sink. Ta Da the problem is at the master. Things are fine hook up the fronts,does the problem come back? Ta Da the problems in the front No ? Hook up the rears,Does the problem come back? Ta Da the problems in the rear.

Ahhh I gotcha, do this with engine running correct ?

 

[Ricko1966]

Ahhh I gotcha, do this with engine running correct ?

Do it j7st like you are now, engine off,then engine running. Start reconnecting things until your symptoms come back.

 

[fast 99]

If plugs aren't available, they can be made out of a brake line fitting. Use a torch and fill the hole with brass.

Or use these to pinch off hoses.

https://www.amazon.com/dp/B002YKHRUK/

 

[squaredeal91]

Those hose pinchers are nifty! I drove around for a week or 2 with vice grips on my front caliper hose until I could afford a new caliper. That's another thought too.

 

[Ricko1966]

If plugs aren't available, they can be made out of a brake line fitting. Use a torch and fill the hole with brass.

Or use these to pinch off hoses.

https://www.amazon.com/dp/B002YKHRUK/

If I don't have plugs I just pound an old line shut with a hammer,fold it on itself pound it shut again. I'm too lazy to break out the oxyacetelyne.

 

[Radiohead]

When you replaced the master cylinder, wasn't there plastic plugs in the ports? Those will work for testing purposes.

 

[Ricko1966]

When you replaced the master cylinder, wasn't there plastic plugs in the ports? Those will work for testing purposes.

Have you ever tried this? I always assumed there was no way those plastic threads would hold that kind of pressure,but never tried.

 

[hoagster]

15 psi, or 1 bar will let you know you have a leak somewhere!

 

[Radiohead]

Have you ever tried this? I always assumed there was no way those plastic threads would hold that kind of pressure,but never tried.

I have metal fittings plugged for when I am doing this operation, but light pressure is all you need to do a quick test, so the plastic plugs should be sufficient.

One thing I've not seen discussed yet is whether or not the fluid level in the master stays the same after several dozen pumps with the engine running. My thought process wonders if the master is leaking out the piston seals and into the booster. Tbe odds are really slim, but it is something to consider if the pressure check comes up empty.