Power brake issue

[B5fourspeed]

4 master cylinders bled brakes 5 or 6 times still have low brake pedal.Front discs will only grab when car is running.When I bled the calipers the pads do not stop the rotor.When I start the car the front brakes will work but the pedal is almost against the firewall.Everything is new plus a rebuilt correct booster.Rear brakes are adjusted to have a little drag.The push rod on the booster has been adjusted with that special tool.My car is 70 Cuda.Car is totally restored.Everything on the car works except the low brake pedal.Should the front calipers grab the rotors when bleeding with the car not running.I don't understand this issue.Any suggestions.

[pschlosser]

Four master cylinders! Wow, that's remarkable.  Is it possible they were all the wrong part?  Were they all the same part number?

A picture of the current master cylinder, installed, in your engine bay would answer several questions about the hook up of the brake lines and proportioning valve.

When I was a young driver, the first time the engine stalled on a Cadillac did I discover how hard one must push on the brake pedal to get  the car to stop.  Is it possible, when the engine is NOT running, you're experiencing the same issue?  On a power brake car, one must press the brake pedal alot harder than a manual brake car for the same result at the wheels.

Any history you can share?  Example: Were the brakes ALWAYS broken?  Did they work before it was stored?  Stuff like that?

[B5fourspeed]

Every part for the braking system is new.The brake pedal is not hard.It is soft and travels to the bottom before the brakes engage.The master cylinders were the correct replacement for that year.The last Cuda I restored I used the same exact brake components bled the brakes once and brakes worked perfectly.The only difference is the supplier of the front disc brake setup upgraded the rotors to drilled and slotted because my order was late.The proportion valve is correct for a 70.If I had a hard pedal with no braking I would think that I have a booster problem.I am missing something with this problem but I have no clue where to go with it.I have no leaks and all 4 wheels are getting fluid.I feel the issue is with the front brakes.Calipers are on their correct side.I will try and post some pictures.Thank you for the input

[Mr Cuda]

I would blame a tripped brake bias shuttle valve.
 Is the brake indicator light on?  Does the brake light come on when you ground the lead off the safety valve?
  I have seen the shuttle stick many times. Usually blocking fluid to the rear, and the rear brake flow sets brake pedal height.
 

[B5fourspeed]

The brake light has never come on.How do I check the valve or reset it? Thanks

[pschlosser]

The brake light has never come on.How do I check the valve or reset it? Thanks

The Hydraulic System Safety Switch grounds the terminal when it detects the warning condition.   To test if your brake warning lamp in the dash is functioning, pull the plug off the safety switch terminal, and ground that terminal to see if the lamp comes on.  I think it also  comes on if the parking brake is applied.

[B5fourspeed]

The light comes on when the parking brake is set.

[pschlosser]

With the engine running, can you clarify the pedal returns all the up when you let off with your foot?  If not, can you pull it all the way up using your toe?  And the problem is when you apply the brake, it travels pretty far, nearly to the floor before engaging the brakes?

All during this problem, have you been using the same booster?

The problem you describe I've seen before, when someone converted from a manual drum to a power disc brake, but was mixing some of the linkage and pushrod parts.

[B5fourspeed]

The booster is a original 1970 correct for my car.All the linkage is correct for a 70 Cuda.The booster was rebuilt by Master power brake in San Jose,Cal.The pedal returns all the way back.The pedals are 70 correct.Yes,the issue the pedal travels almost to the floor before the brakes engage.I plugs the ports to the master cylinder and my the pedal was hard.What concerns me is no matter what I do the pedal feel is exactly the same.Should I disconnect the vacuum to the booster and see if I get a hard pedal.I think that this is one of those issues that could have 10 causes.I am asking for ideas before I tear things apart.I know it will come down to 1 thing that is wrong but it's  just finding that 1 thing.Can air bubbles still be in the system somewhere? Thank you for your responses.Let me know your thoughts on tackling this problem.

[pschlosser]

Can air bubbles still be in the system somewhere?

Only you can answer that question.  We're not there to supervise and look over your shoulder.

Not everyone knows, but the reason hydraulic technology works so well, is liquids don't compress.  But air surely can.  What you describe is very much how air in the brake lines or master cylinder would behave.

If there WERE air in the system, as you depress the pedal, the air compresses but the brake fluid doesn't move, the pedal moves closer to the floor.  Once the air hits a certain high level of PSI, only then does the brake fluid move and cause the rear brake cylinders, and the front calipers to move.

If you were doing something during the four master cylinder installs to cause this, they may all do the same thing, if you're installing them the same way each time.

Are you bench bleeding the master cylinders, before hooking them up to the brake lines?  Are you bleeding the master cylinder attached to the booster by pressing on the brake pedal?  Or actually bleeding them on the bench fully depressing the piston until all the tiny bubbles are gone?

Then after, when you're bleeding the brake lines at the brake cylinders of each wheel, are you starting with longest brake line (typically the right rear wheel) first, then moving to the next longest (rear left), the right front, and lastly the left front?  Are you seeing air come out of each brake cylinder/caliper when bleeding?

Is the engine running when bleeding the brake lines?

How much total brake fluid do you end up bleeding from the brake line system, after you install the bench bled master cylinder?  If it is a small amount,  like 1/4 to 1/2 cup  (2-4 fluid ounces) you may not be bleeding them enough.  I  think it takes  at least 8 ounces or more to fully bleed the brakes on a master cylinder change.

When the fluid comes out of the brake cylinders/calipers, is it clean?  Or is it dirty?  Do you see  any brake fluid leaking from anywhere?  Is the brake fluid level in the
 master cylinder going down?  Do you need to keep topping it up to keep it full?

There are no right or wrong answers.  But your answers may help me and others figure out the problem.

Does anyone know, can a faulty brake booster cause this issue to occur?  I'm thinking, "no."

[B5fourspeed]

Thank you for the reply.This last time I bled the front brakes I gravity bled them and the right front caliper did not have a lot of fluid out after almost 2 hours of bleeding.I always bleed starting with the RR,LR,RF and LF.I don't see any leaks.Should I bleed the front brakes with the car running? Over the course bleeding the brakes 5 or 6 times over this last year I would get a decent amount of fluid coming out.I think the issue is with my front brakes if I was to guess.

[pschlosser]

Bingo. Gravity bleeding.  I bet that's your problem. 

Air can still be trapped in the system unless you got some pressures going on.  Not always, but it CAN be trapped.  Get a helper and bleed the brakes by the helper pumping the pedal on your command.   When you're ready to stop bleeding, tighten the bleeder screw back up, before the helper releases the pedal on the last pump.  Some people like to tighten the bleeder on each cycle of  the pedal being released.

Still use the bottle with the tube from the bleeder submersed in fluid.  Observe for yourself and watch for the fluid to run clear (presuming it's not already)

I would be surprised if you don't see any new air bubbles doing it this way.  And since you don't want to repeat this, yet again, I encourage you to bleed 1-cup, or 8-ounces of fluid from each wheel.  Do that, and I would feel pretty confident you no longer have air in the system.

With a helper,  you can get them all bled in under 20 minutes.

Report back any weird things you observe.  For example, no fluid coming out at all at one wheel, or something else different than the other wheels.

If fluid doesn't flow when pressure is applied at the pedal, we know what to do next. You could have a flexible brake line in need of replacing, or a clogged proportioning valve.  Gravity Bleeding won't make a blockage obvious.  2-hours to bleed the system is a VERY long time.  You may have a blockage issue.  But first thing's first.  Bleed the system under pressure.

[B5fourspeed]

Thank you very much for your input.All the lines and hoses are new.I will bleed the system again per your instructions and get back to you in 2 or 3 days

[71GranCoupe]

Lots of ways to bleed brakes on you tube. Many show how to do it with just one person with a bottle and a hose out of the bleeder. Key is to make sure you have enough fluid in the bottle so when you release the brake pedal no air returns back into the lines. After a few pumps, add more to the master cylinder so as not to have air enter the lines at the MC. I used this method on my motorhome and it worked great.

[pschlosser]

Thank you very much for your input.All the lines and hoses are new.I will bleed the system again per your instructions and get back to you in 2 or 3 days

Good job.  I'm not saying what you did was wrong.  But considering the pedal acts like there is air, and if you're using  gravity bleed all those master cylinders, I suspect that is the issue.

If you examine the tube on an I.V. bag feeding saline to a patient, you may observe there are air bubbles in some of those tubes.  Air in the blood stream can cause a stroke.  But air in the I.V. tube is nothing to panic about, when the flow from the saline bag is slowly dripping.  What we observe is the fluid flowing around the air bubbles in the tube, while the air bubble stay put and don't move.

For some brake maintenance actions, where you simply want to dump the old fluid for new, or remove any air pockets at the very end of the circuit, where the brake cylinder/calipers reside, gravity bleeding is a decent option.  Example, replacing the shoes/pads and a tiny bit of air enters the cavity.

But when replacing the master cylinder, air may be trapped at the beginning of the brake line circuit.  Slow flowing brake fluid may not move the air bubbles outward to the end, just like in the I.V. example above.

I hope this proves to be the cause, and well done @B5fourspeed for being patient with all my trivial questions.