Not so fast at pointing fingers. Remember an earlier thread, you had both front & rear brakeline ports plugged off on the MC and the MC held pressure, firm pedal. I think the issue lies past the MC.
The trouble with using a aftermarket special instead of the correct MC is not only the bore size may be incorrect, but also the distance (gap) between the back of the MC piston and the booster rod. Some rods are adjustable, some are not.
The experts say a gap of only 0.020 is needed, no more. With no gap at all, the brake pads are constantly engaged. With too big of gap, you get excess pedal travel. There is a special tool to measure that gap but IMHO it's not needed. By removing the MC from the booster, a piece of putty can be used to see if things looks normal in a blind spot.

 

I'm grasping at straws.

I'm getting no bubbles bleeding ANY of the calipers. Im convinced it's flushed out. It just isn't holding pressure.

 

So, let us digress:
Brake pedal to the floor.
New Master holds pressure when tested solo.
All 6 bleeder screws show no bubbles.
New Prop V.
Rubber lines good.
Cue the Jeopardy theme song.

 

I'm borrowing a vacuum type bleeder from a friend today.

New Delco master cylinder will be here this afternoon.

Back to the auto parts store for more brake fluid.

Then back to square one.

 

Do you have the kit for bench bleeding the MC?
(see my profile photo album)

 

Oh yes.

I've been to this rodeo many times.

 

I received a new Delco mc yesterday.
Bench bled the mc in level, tilted forward, and tilted backward angles.
Borrowed a MightyVac vacuum brake bleeder from a friend.
Bled the brakes twice around using the vacuum bleeder.
Bled the calipers in the order in the GM Shop Manual.

SOFT PEDAL WHICH SINKS TO THE FLOOR.

So I went around for the third time bleeding from the farthest to the closest caliper to the master cylinder.

SOFT PEDAL WHICH SINKS TO THE FLOOR.

4th go around today I bled the rears only for a long time. Checked the pedal and...

SOFT PEDAL WHICH SINKS TO THE FLOOR.

I got frustrated and walked away from the project for the day. I guess I'll mess with it again tomorrow. I'll use the Motive Bleeder this time. You can see the air bubbles more clearly than with the vacuum bleeder.

Back to the auto parts for more fluid.

 

You know what they say about doing the same thing multiple times and expecting different results ....

Since you have changed the master cylinder out - back to square one - plug the output ports on the master and see if you have a good pedal. If so - pull those plugs - put the plugs on the lines right before the calipers and see how the pedal feels. Then hook up the front calipers and block the rear line and see how that is. Then do it the other way around - put the plugs on the front lines try to block the fronts and hook the rears up.

You have to figure out where the problem is... You either have air in one or more lines - a leak / bad seal or something is moving significantly more than it should be. One other test to try - when the pedal goes to the floor - do a very fast release and reapply - does the pedal "pump up". If it does - it's a sign that there is either air in the line or something is moving further than it should. If it doesn't seem to "pump up" - I'd bet more on a leaky seal somewhere....

You were smart to walk away - believe me - everyone who turns their own wrenches has gotten frustrated at stuff like this before. IT happens !!! Don't let it get to you - when it starts to get super frustrating - walk away - and come back later. Fundamentally - this is simple hydraulics - there is an answer and you're GOING TO find it !!!

 

Thanks for the words of encouragement and the solid advice Purple92.

 

Hang in there man!
I’m anxiously awaiting your victory because I need to tackle my soft pedal next weekend.

 

BTW - Don't think for even a second that this stuff doesn't happen to Pro's as well. It does - and this kind of stuff is (at least partially) factored into the prices they charge. And even with factoring stuff like this into the cost - when it happens - it can really ruin a week ($$$ wise) for a pro.

 

Today I blocked off the ports of the mc. I discovered that I have a hard pedal when both ports were plugged.

Saw a very very small leak on the ground under the left rear caliper. Pulled the caliper off. I discovered that an inner piston seal had been leaking on the backing plate as the paint was destroyed.

My next issue...
I have SSBC rear brake calipers. I don't know if the factory seal kit will work on these calipers. Just to cover my *** I ordered some factory type seals. Then I discovered the kits for SSBC calipers at Summit Racing. Ordered a kit and of course it's Friday so I lose two or three days of work time.

The saga continues.

 

Today I took the left rear caliper apart. There was evidence of a leak on the left rear inboard caliper piston seal. I'm not saying this is the solution to my problems yet.

I'm sceptical that the front will now have a problem when bleeding them. It seems that I saw a very small drip of brake fluid below the newly installed combination valve. I'm receiving a SBBC caliper rebuild kit from Summit sometime today. So I'll get that caliper reassembled and installed. Then on to more bleeding.

 

GOOD !!! You're making progress !!! Remember - It does Not take a big leak to cause a brake pedal to fall...

While you're under the car having fun - try giving the lines to the new proportioning valve another 5 degrees of tightening.

Odds are decent that between the two issues - you have found your problem.

 

Need a better definition of Sinks to the floor.

If a brake pedal sinks with just a light pressure on it, that is a sign of fluid bypassing the pressure seal in the master cylinder.

 

Hey guys,

Quick question before I start this process next weekend.
When you plug the MC to test the pressure without the lines, how do you keep from introducing yet more air in the lines.

 

You can't, that's the thing. No matter how quickly you remove the plugs and install the steel lines, you have introduced the dreaded bubble of air into the system.
Thats why when working on the booster rod for example, you try to leave the brakelines still attached to the MC if possible.

 

If you have a helper available - you can try having them slowly depress the brake pedal while you install the plugs, but that isn't a guaranteed way to solve the problem...

 

Pretend the plug is a bleeder, and use them to bleed the master after you screw them in.

 

Huh? Lost me there.
O.P. is saying if when the MC is installed on the booster, fully bench bled with plugs in the brakeline ports, will air be introduced when the plugs are removed and front / rear brakelines are then attached.
Yes. No matter how quick you are. Because the brake fluid is not likely at the top of the brakeline flare & fitting. (air pocket)