Disc brakes feel spongy no matter how much I bleed them

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cudajim

cudajim

cudajim
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My 67 ragtop Barracuda has a set of 73 front disc brakes and stock 10" rear brakes. I am also using a new manual brake master cylinder. I got the entire system off a donor car including the proportioning valve, lines etc. I also have new rubber brake lines. Yesterday, I took the master cylinder off and bench bled it with 2 short brake lines feeding over the top. It didn't help at all. The caliper bleeders are at the top and I've bled the system several times. I get poor braking, cannot lockup the wheels at all. Any suggestions?
 
Got to ask... Have you driven a manual brake car before? The feel is different.
 
Rear brakes adjusted correctly?
I usually set them up so they are
Just rubbing. And cycle pedal several times while adjusting. parking brake cable needs to be adjusted properly too.
disc brakes use a 3 psi residual valve and drums use 10 psi residual valve.
 
My point is if you are used to power brakes manual can feel like they are not going to stop
 
What fluid? DOT 5 can do that.
 
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When i converted my duster over, I called Cass from Dr Diff. He made sure i got the correct master bore size and correct proportioning valve for a manual disc drum setup. My setup feels good car stops fine.
 
Did you replace all of the rubber hoses to include the one on the rear axle housing? Also if you have the wrong MC, your brakes wont work well. How about the distribution block and proportioning valve? Any blockages?
Also, what bleeding technique do you use? I have a friend who does the pump, pump, pump then hold pressure on the pedal while someone opens the bleeder screw. I don't care for that method. I prefer the open bleeder, pedal down, close bleeder, pedal up, repeat method. I have had much better luck with that.
 
Tooljunkie said:
Rear brakes adjusted correctly?
I usually set them up so they are
Just rubbing. And cycle pedal several times while adjusting. parking brake cable needs to be adjusted properly too.
disc brakes use a 3 psi residual valve and drums use 10 psi residual valve.
Click to expand...

That's a good point, you can also adjust them up tight to see if the pedal is low because of shoe adjustment. So far as residual, tho, these cars did not use and should not need one in the disk circuit. The drum end, (rear) yes. That should be built into the proper master
 
Bodyperson said:
What fluid? DOT 5 will do that.
Click to expand...

Uh, NO. Harley never had that problem and ran DOT5 for three decades. Properly-bled DOT5 will NOT do that.

OK OP, time to do yer homework.
Install master on car and bench bleed. Do not hook up lines. Instead, cut some pop can metal and place under the lines, and then screw the lines down, so no fluid can flow. I think you can get dead-head caps for this, too. Pump the brakes. Firm? If yes, keep looking (down to the next paragraph). Not firm, even after a bench bleedin'? Replace master.

Hook up the front brakes only and bleed. (Make sure you're hooking up the front system, with the disc brakes, and not necessarily the front line on the master cylinder). Pedal firm? If so, it's a rear brake problem. If mushy, it's another issue. You can keep going down the line.

It might be a hose going bad.

If the fronts keep a firm pedal, the rear brakes might be misadjusted, etc. but at least you've narrowed it down to a system. You can even narrow it down to a corner this way, if you want.
 
also, if the brake pedal is not firm, sinks slowly to the floor with foot pressure,
Do a close inspection of each brake line fitting for a very small leak.
A very tiny leak, a very tiny drop is enough to cause the pressure to fall off. Wipe each fitting with a dry paper towel, pump the brake pedal, go back and re check the fitting with a dry towel. If nothing is found go back and do the same test on each hard and rubber line.
 
One last thing. It wasnt mentioned nor asked. pre- bent lines?
You did lines,bending/ flaring yourself?
Reason i asked is a customer had a similar problem after replacing a line, made a big coil to avoid cutting and flaring. Loop trapped air.
 
I have not tried this yet but others have said to gravity bleed the front disks. open both front bleeders, and keep the rear ( larger) cylinder full as it drains to the disk bleeders. close the disc bleeders when no more bubbles come out. some clear hose on the bleeder is really helpful (can not do this on rears due to residual pressure valve.)

I have also found that by pressing the pedel down too fast during bleeding can almost aerate the fluid. it took me a week of bleeding to learn that lesson.

Good luck.
 
Need to be clear on the symptom.
If the peddle stops at some point and does not bleed down, that tells me the system is not leaking.
If it is "spongy" I take that to mean it does not have a solid feel but it does stop at some point.
If this is the case, I would suspect air in the line as the most LIKELY cause.

Do you have an air compressor?
 
Proportioning valve is stuck? Or the Master cylinder push rod to long or short? I had a car come in that left they the shipping tab on the master cylinder instead of installing the ring and boot if equiped with one. Instead of a C-clip. The piston was relying on the brake switch and was coming out to far. You need to diagnose and fix this yourself. We are all just guessing without inspecting your work and or parts used.
 
I appreciate all the comments. I am using dot 3 fluid, yes I am used to manual disc brakes, I did a good bench bleed, I have replaced all 3 rubber hoses etc. I will try today to block off the master then the rears then bleed the front again to see if I can nail down the problem. I have the stock master and did not take the master apart to verify the size of the bore but it may come to that.
 
did you "bed" the new pads?
Try taking the car some place safe and haul it down hard from 60 mph to 30 mph 3 times...let cool and haul it down from 30 to zero 3 times.
 
cudajim said:
My 67 ragtop Barracuda has a set of 73 front disc brakes and stock 10" rear brakes. I am also using a new manual brake master cylinder. I got the entire system off a donor car including the proportioning valve, lines etc. I also have new rubber brake lines. Yesterday, I took the master cylinder off and bench bled it with 2 short brake lines feeding over the top. It didn't help at all. The caliper bleeders are at the top and I've bled the system several times. I get poor braking, cannot lockup the wheels at all. Any suggestions?
Click to expand...
I had the same issue.
Get rid if the rubber flex brake lines and replace them with stainless steel braided lines from a US supplier. The rubber lines balloon under pressure. Changing out the rubber lines helped my brakes allot I hope it does the same for you.
I also went with a very high quality pad that I also made a noticeable difference.
 
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