Brake Wars..Episode 3.. The mechanic strikes out..again

[midnight sun]

Is there any chance the brake line runs up and then down again so that an air bubble could be trapped in the line with no way out?

Sitting here late at night at the computer Im running that over in my mind.  Recollection is that the line is a straight shot out of the M/C to the drivers side T.  By straight shot I mean no ups or downs.  There are a few lefts/rights.  From the drivers side T the line goes down about 1 inch across the front crossmember and then back up about 1 inch to the passenger side through frame fitting and then up and into the caliper.  Based on that, I would have to say no to the question.  Having said that, I will check just to make sure tomorrow.  We are leaving for the Pueblo Nationals on Thursday but when I get back I am going to try your suggestion with the aluminum cap and Schrader valves.  Certainly cant hurt anything.

[Bruce Dorsi]

...it still puzzles me why they will bleed good separately but when everything is hooked together I lose some of the good pedal.  Almost like there is not enough pressure/fluid to activate both calipers at the same time.

Jack, let's look at this from another angle.  

If your master cylinder is too small, you will not displace enough fluid to move both pistons out far enough to create the firm pedal.  

Although a small m/c will generate more hydraulic pressure (other factors kept equal), there will be less fluid moved in the circuit.

Removing the calipers from the circuit can create a firm pedal, because very little fluid is displaced from the m/c.

Having one caliper in the circuit could allow the pedal to remain firm because there is sufficient fluid flow to properly operate that caliper.

Connecting the second caliper, and losing the firm pedal indicates insufficient fluid available from the m/c, or excess air in the front circuit.

So there are a few possibilities:

1.) Unbled air in TWO calipers will compress more than unbled air in ONE caliper.

2.) The displacement of the m/c is insufficient for your calipers.  ....This can be caused by: a) to small a BORE, or  b) insufficient STROKE.  

If the pedal travel does not allow for FULL travel of the m/c piston, in both directions, your problem lies in the pedal linkage or the adjustment of the rod between the booster and m/c.

If the m/c piston can travel to its full extremes, then the bore may need to be larger to displace more fluid.

Do you know what size bore your master cylinder has?

Are your caliper seals the type that retract the piston further and require a quick take-up master cylinder, which has a larger or stepped bore?

Is the m/c piston retracting all the way back against the retaining ring when the pedal is released?

Sorry, I know there are more questions here than answers.

Rather than guessing at everything, using a high-pressure guage to troubleshoot, may be beneficial.

If you temporarily tee a guage into the front circuit near the m/c, you can check the circuit pressure with both calipers blocked off, then one caliper connected, then both calipers connected.  .....Peak gauge pressure should be the same in all three scenarios. ....If the readings drop from test to test, I doubt your calipers are at fault.

[Leon]

If the pedal travel does not allow for FULL travel of the m/c piston, in both directions, your problem lies in the pedal linkage or the adjustment of the rod between the booster and m/c.

Is the m/c piston retracting all the way back against the retaining ring when the pedal is released?

Thinking about this issue your post reminded me of a system I had to troubleshoot and found the adjustment on the booster didn't allow the piston to return fully and gave a soft pedal.  We took the master off, adjusted the rod on the booster, and the pedal had the proper feel, brakes worked fine.  It can be any of several of the above items, just have to ensure each is checked.

[C9]

Thought I'd toss the too much vertical up and down in the line in since that can cause problems.
Heard that a street rod builder in the old home town couldn't get a system to bleed properly and they ended up hanging the car from an engine hoist so it was at an angle.
The thinking was, the air bubble would rise overnight and be easily bled the next morning.
Never heard if it worked or not, but with your lines pretty much on the horizontal it doesn't seem to be a problem.


Bruce and Leon have some good points.

It doesn't look like you have insufficient - or no - brake pedal freeplay, but it is something to check.
If there isn't enough - or any - freeplay the piston can't retract far enough to admit 'make-up' fluid into the system.

Master Cylinder bore may be part - or all - of the problem.
As a fwiw, the M/C bore in my 32 is 7/8".
It works well with the 68-70 era Mustang disc brakes and the 68 Merc wagon drum brakes on the nine inch rear axle.
No power assist, just plain ol brakes, but force required at the pedal is easy and imo these small cars do not require power brakes.

I'm guessing the Mustang calipers take more fluid than do the calipers on your car so perhaps M/C bore capacity isn't too critical for you although it is something to think about.

What do you think about going to a brake shop and having them bleed the brakes with their power bleeder?

[midnight sun]

What do you think about going to a brake shop and having them bleed the brakes with their power bleeder?

I considered that but doubt that I will for a couple of reasons.  First, I doubt that they would want to fight accessing and getting the top off of the M/C.  Its a real hassle.  I originally put in a access door but due to my inability to  see too far into the future, I didnt take into consideration that I would have to remove insulation, carpeting, seats to get at it!!  So in lieu of that you have to do it from under the car in a very tight space.  Also for the same reason, I doubt that their equipment would fit in the tight space between the top of the M/C and the floorboard.  Theres only about 3 inches clearance.  Having said that, when I get back from Pueblo, I may just go down and talk to them and see what they say.  Cant hurt to talk to them I guess.