Front Brakes Don't Work - Need Help!

[Captain Obvious]

Of course us on-line are limited to the descriptions of the behavior, and because of this things can get murky. At this point, you're saying you've bled all four corners and they all look great except for the right front. And at that right front, you've got a very weak stream of fluid and a pedal that still goes to the floor. Other than that, everything seems to be OK. Is that correct?

One thing to keep in mind is that it really doesn't take a lot of pressure in the lines to lock up a corner (any corner) to the point where you can't turn it by hand. In other words... Just because you can't spin a rotor or a drum by hand doesn't really mean you've got normal pressure in that corner of the system. Even a small amount of hydraulic pressure in the lines is enough to prevent the corner from being able to be turned by hand.

Hope your brain clearing walk-away provides some ideas and you come back shortly with the answer.

[Dens240]

I agree it's somewhat of a struggle to convey all the variables that I've tweaked during this troubleshooting activity. Your assessment of the current state is accurate except the problem child is the left front and not the right front. 

I get what you're saying about whether I can spin a rotor by hand. I really won't know about true braking performance until I back it out of the garage - I've done that before and then pulled right back in because the brake performance was so poor. What I can say is that on the right front, I have gone from being able to freely spin the wheel by hand to now not being able to move it at all by hand, so that is at least progress in the right direction.

At this point, I'm mentally preparing to start the entire brake bleeding process over from scratch. Something I'm not excited about, but may be necessary.

What I've done to date:

• Checked protrusion of brake booster rod vs depth to engagement with MC piston. Based on my measurements with a caliper, they should be within a few 1/100s of an inch when the MC is mounted.
• Adjusted the brake pedal range of motion to maximum travel to ensure the MC piston is able to fully recede to its original position after the brake pedal is released
• Bench bled the MC (literally on a bench). I think this action helped "prime" the entire system and seemed to have a positive impact. I remounted the MC while still full of brake fluid - a difficult job while simultaneously ensuring no fluid gets onto the paint. 
• Corrected a brake fluid leak at the inlet to the brake warning switch that mounts just below the MC. I ended up building a new line from the MC front brake circuit to the warning switch. No leak at that point sense. 
• At this point in the process, I was successful getting the right front brake to fully bleed and I now get brake performance (when tested by turning the wheel by hand). I still did not have any brake engagement on the left front. When bleeding the left circuit, there is limited fluid flow out of the bleed valve. Note that the rear brakes have consistently worked and I get really solid flow each time I bleed them.
• I then worked exclusively on the left front brake circuit. I confirmed that fluid exits the brake warning switch when the pedal is depressed, although I don't have a good means to measure the flow or force as it just sprays out of the port (into a small bucket I was holding)
• I checked that I could "blow through" the hard lines in the left front circuit and I replaced the flexible hose with a new hose. 
• I was not able to blow through the entire left side circuit (from the line at the warning switch to the caliper bleed valve. This made me suspicious of the (brand new rebuilt) brake caliper and I removed it from the car and took it to the bench.
• I set it next to the original (also rebuilt but by me) brake caliper and compared my ability to blow through each from the fluid inlet to the bleed valve. The original was easier to blow through. I suspected a blockage in the newer caliper and used my brake bleeding vacuum kit to pull on each at the bleed valve and compare the vacuum. After a few tries, the newer caliper started to behave in line with the original possibly indicating an obstruction had moved but I didn't see anything.
• Still suspicious of the caliper, I installed the original, hooked it up, and proceeded to bleed the lines again. No change. Still very low flow of fluid out of the bleed valve. I removed the original and reinstalled the newer caliper (my preferred if I can get it to work).

That's where I am as of today. Parts that have been changed during this process:

• Master cylinder
• Brake warning switch 
• Proportioning valve (mounted under the rear of the car on this 1972)
• Both front calipers
• Brake line from MC to warning switch for the front circuit
• Brake hard line from the warning switch to the flexible hose on the left side

I'm glad you got me thinking through this as I knew I needed to get all this documented for posterity!

[Zed Head]

Maybe this will help, below.  I see flow and pressure and force being used all in the same train of thought.  Flow is what starts things happening, but once flow stops you're left with pressure.  And pressure is what creates the force on the rotor that stops rotation, via friction from the pad on the face of the piston.  If you know that you have flow when you press the brake pedal, when the brake pedal stops moving you can stop thinking about flow and focus on pressure.  

Once flow stops you have to have equal pressure throughout the system.  It's a law.

https://en.wikipedia.org/wiki/Pascal's_law

[Dens240]

And THAT is what has me so perplexed by this (my background is in mechanical engineering so I am familiar with fluid mechanics/dynamics although it's been a few decades). Sorry for mixing terms but let me try to clarify.

Regarding flow - I use the term flow when I have a bleed valve open and the system is not able/allowed to build pressure at that wheel. When I bleed the rear wheels and the right front, I get full flow meaning the clear tube attached to the bleed valve is full - all surfaces of the inside diameter are in contact with fluid. When I bleed the left front, the clear tube attached to the bleed valve just has a trickle of fluid and sometimes no fluid. Like a small stream in the middle of a dry/hot summer (I'll avoid my earlier swollen prostate analogy). In one of the earlier posts, I captured a video of this.

Regarding pressure at each wheel. With the circuit closed so that pressure is allowed to build, I get brake adhesion at the right front and both rear wheels but nothing at the left front. Shoes and pads are making strong contact with the drums and rotor.

It appears that the front brake circuit should be common from warning switch inlet from the MC so I struggle to understand how one front brake can work and the other cannot without a solid obstruction on one side - and I cannot find any evidence of an obstruction or when it seems I may have some evidence, I've swapped out the suspicious part.

[Mark Maras]

 At this point I'd disassemble the caliper.

[Patcon]

I think I might also. Make sure all the passages are clear

[Dens240]

It appears to me that the front brake circuit is essentially common from the point where the brake fluid enters the warning switch. Below is a marked up image showing what I think to be true. If true, it seems the only possible cause for what I'm seeing is an obstruction in the left front circuit. I think I'll look at the warning switch next.

 

[Zed Head]

Edit - I wrote the below while you posted.  

Another simple check of just the lines and system up to the calipers would be to disconnect both calipers and place the ends of the lines in two containers.  Sit in the drivers seat and pump the pedal a few times (don't bang the MC piston off the end of the bore, use short strokes).  If the lines are open and pressure is fairly balanced you should get about equal volumes in each container.  If you don't then you can just go backward toward the MC.  Disconnect at the switch and make some stubby lines to check flow there (you have a number of old pieces laying around by now).  Do the same test.  Since there's only a single line for the fronts to the switch there's no reason to go above the switch.  There's no left-right up there.

That will break the system up in to three parts - the calipers, the lines, and the switch.

Edited November 17, 2021 by Zed Head

[Dens240]

I like it! That's a solid design of experiment and I think I'll do it. As it turns out, I have a few extra brake lines lying around...

[Patcon]

I wondered if you could move the flex lines up to each side of the switch along with the brake calipers. Put a block of wood in the caliper to prevent blowing the pistons out. then see if they both build pressure there

[Captain Obvious]

Saw a mention about having a clogged bleed nipple on a different thread:

https://www.classiczcars.com/forums/topic/65237-barefootdans-280z-build/?page=7&tab=comments#comment-631700

That would give you all the symptoms you're seeing, but it would be hard to believe that a) the nipple on your freshly rebuilt caliper is clogged, and b) the nipple on your old original caliper has the exact same issue.

Not saying it's impossible, but certainly unlikely? In any event, it may be worth the five minutes it would take to find out.

[Dens240]

Hopefully the final entry in this thread! Early this morning I went through the following steps.

• Disconnect and remove the left front brake caliper
• Confirmed brake fluid flow back through the lines to the warning switch by disconnecting each connection.
• On the bench, I removed the brake pads from the caliper, put a board between the pistons, and used compressed air to push the pistons out a bit and confirm the they could move freely.
• Reassembled and reinstalled the left front caliper and hooked up the lines
• Bled the master cylinder
• Bled the right front caliper
• Bled the left front caliper

And now the brakes work! I'm guessing the compressed air on the caliper pistons was the likely thing that changed course since that's the only step I hadn't done in the past. I was able to do some driving this morning and now I need to work on the idle. The carbs were rebuilt by Z Therapy and I need to spend some time balancing, etc. 

Thanks to all for the suggestions and help.