Old master cylinder was leaking like sieve. Replaced with a new one. Bench bled the MC. Bled the lines. Pedal gets hard and drops down about 3/4s. When i turn the engine on the pedal gets real soft and goes all the way to the floor. What could be the issue? I've Bled the lines 4 times.

 

You could check the internals on the master cylinder. Sometimes "new" doesn't mean "good".

 

How true.
I replaced master & booster and never really had a good pedal.
Replaced Ebrake cables and pedal went to the floor and nothing I did made it come up so replaced master.
Pedal is great now other master was only 2 years old.
Dave ----

 

The hard pedal at first and super soft when the engine starts just means that the booster seems to be working correctly.
But the hydraulic system is either completely out of adjustment or there is still a lot of air in it.

The first place I would look is an area that is always seeming to be a problem with new masters, new boosters, or both. That’s the adjustment between the booster rod and the back of the master cylinder.
They must be almost touching or you will get the long travel of the pedal like you have. There is a specification but you’ll have to look it up or someone might have it.
It’s a very tiny gap between the rod and the back of the piston.

You probably don’t need to remove the brake lines yet. You should be able to pull the master cylinder slightly forward and see what you’re dealing with.
Look in the back of the new master cylinder and see if it is a deep well or a shallow well.
Back in the day most, if not all manual master cylinders had a deep well with the pedal rod stuck into it. Most booster master cylinders had a shallow well so that the short rod from the booster did not need to extend far.
Nowadays some masters are deep but come with a spacer to fill the gap.
Even still you usually need to adjust the rod to match the master.

 

This is the rod from booster.

Rod in booster.


This came with new master

Rod with rubber piece from new MC. Is the spacer you're talking about?

 

I don’t see the piece I was talking about.
Looks like a little bullet shaped thingy. Often with a little rubber or ring to keep it in place.
Was that last picture of the booster supposed to be the picture of the insert?

 

First picture is of the rod that goes into booster..

 

Last picture is of rod with round rubber piece from new MC in the booster..

 

That rubber piece is just a dust boot and has nothing to do with the fitment of the rod to the back of the piston in the master cylinder.
What you need to find out is if there is a gap between the booster rod and your new master.
If there’s a gap then the pedal travel will be immense. At least six times larger than the gap.
For example with a pedal ratio of 6 to 1 a 3/8 inch gap between the rod and the piston would equal over 2 inches of pedal travel.

This is a very serious adjustment that needs to be performed anytime either a booster, or a master is changed.
Specification for it is something like less than 1/64th of an inch. Probably better measured in thousandths of an inch.
It’s important because if it’s too tight the brakes can stay engaged and get worse as things heat up. Locking things up.
If it’s too loose you experience excessive pedal travel.

Did you happen to get a picture of the back of your new master cylinder and maybe even one of the old master cylinder?
That would be a great way to compare any differences.

 

These came with the new MC. Are these the bullet rods you were talking about 1TonBasecamp?

 

No, those are standard pedal rods for different manual brake setups without a booster.
The ball end with the black clip snaps into the deep well of the master cylinder.
A deep well that might not be compatible with your booster rod without the spacer I was talking about.
The bullet thing I was talking about is approximately three-quarter inch long with a pointy tip at one end that goes into the master cylinder and a concave outer end that the booster rod fits into.

Since the brakes don’t work you might as well remove the master cylinder from the booster and have a looksee at the back end.
Take a picture so we can see too.

 

Pictures of old MC