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.

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...

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.

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.

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!

I removed the bent s line as well - disconnected at the caliper. I’m going to take a couple days off from putting hands on the hardware and possibly start over from the MC in a few days and work my way back out to the wheel…unless I get a good suggestion from someone.

The right side is now working. I had a small leak at the inlet to the pressure warning switch for the front brakes. Once I replaced that inlet line and no longer had the leak, the right brake started working fine (after bleeding). I still don't get the same performance on the left. Are you saying that it's possible for one or both pistons in the caliper to be retract too far and block the fluid path? This doesn't seem possible and would be a pretty poor design. But...I could put air pressure on the caliper inlet and see if I can get the pistons to move some, then try the whole thing again.

Latest...I changed the flexible hose - no change. Disconnected all lines between the warning switch and the left caliper and confirmed I can blow through them without any issue Removed the caliper and set it up to try to blow through from the inlet to the bleeder valve. It was difficult so I set up a vacuum on the bleeder to see if I could do better. Nothing. So, I put on a different caliper (yes - I have quite a number of parts on the bench as I'm trying to chase down this issue. I mounted the different caliper, hooked it up, and...no improvement. I'm about out of ideas on this. I've changed everything between the MC and the caliper including a replacement pressure switch that I got from @zKars. Again...I get good brake engagement at the right caliper but not the left. This has be sufficiently perplexed that I may look for an actual professional to diagnose the issue!

This is the tail end of a 4 year restoration. Nearly everything else is finished. It's ready to go...I just need to be able to stop once it goes. So, this isn't a random issue that cropped up. There is something not right with the brake reassembly. I should have a new hose today then will get it on the car in the next day or so.

I did blow through the hard line from the switch to the hose but I haven't blown through the remainder. Also, I built a new line that goes from the switch to the hose and installed that as well with no improvement. I'm picking up a new hose tomorrow just in case and then when I get some time later in the week, I'll take the rest of that circuit apart. At least there was positive direction achieved today with the right caliper!

Cap'n - yes I bench bled the MC, which did improve things at that point. Prior to bench bleeding, I couldn't get fluid flow at either front caliper. After many adjustments - bench bleeding the MC, double checking the spacing between the booster rod and the MC piston, adjusted the brake pedal travel to ensure the MC piston can run through its full range of motion, built and installed a new brake line between the MC front brake reservoir and the warning switch - I was able to get a good full flow of fluid at the right caliper and I get good grip of the pads against the rotor. However, I still only get a very low flow of fluid at the left caliper. It's like an old man with a swollen prostate. The video I posted was taken with the speed bleeders but it would look the same with the replacement bleed nipple and the helper method. I'm now working my way through the lines from the brake warning switch to the caliper to see if I can find an air leak.

I agree. I'm suspicious of the lines from the switch to the left caliper. I have made new hard lines today with no improvement. I've just ordered a replacement hose to see if that's the culprit. Interestingly, all hoses and lines were new. I should have just made my own lines from the start. I replaced the speed bleeder with a standard bleeder and went to the old helper method with no improvement. Unless someone posts a eureka idea, I'll probably wait now until the new hose arrives late this week.

IMG_6939.MOV Latest update...I now have pretty solid grip on the right side caliper. Still not getting any grip on the left. The video below will show that when I pump the brakes, I'm not getting real solid fluid flow. I'll start looking for another leak in the left front lines. Nothing obvious so far. Hopefully you'll be able to zoom in on the video. Again, I'm using speed bleeders.

Even with the good flare, I formed another piece this morning. Homemade on the left, original on the right.

Flare looks fine