Need help confirming or correcting my poor braking diagnosis.

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Joined
Jan 20, 2022
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Location
Homer Glen, Il
My Car
1971 Mustang Mach 1 351c FMX trans and 9 inch rear end
I have been having trouble with my brakes on my 71 Mach 1 Build. It is a stock front disc brake power brake set up. The pedal just is way too soft and it barely stops. As I am reassembling the car I installed a new master cylinder, rebuilt power booster, all new brake lines, all new brake hoses, rebuilt front calipers, new rear wheel cylinders, new proportioning valve. I assembled and filled with DOT3 fluid and bled and could not get the pedal to firm up. I tried bleeding the system again and did not see any air in the bleeder lines, still no firm pedal but all four wheels stopped in the air when I spun them and my son hit the brake pedal. I tried bleeding once more (again no air detected) and then took the car for it's maiden voyage and the brake pedal was soft and the car did not stop well at all. I used a reservoir to collect the brake fluid at all four wheels but dumped it out after each wheel. I looked into the master and it looked like there was some dirt in the bottom corner. I checked the bleeder reservoir and the fluid from the left rear wheel cylinder had some debris in it. I am not sure if the fluid from the other three wheels also did as I emptied the reservoir after each wheel. I don't know where this came from when every single part is new and the lines were flushed out before I installed them and the fluid was in brand new bottles that I cracked the seal on. I have resigned myself to the new master cylinder is defective. And I am hoping the debris in the reservoir isn't a big deal. My plan is to remove the master and vacuum out the fluid from the system re-flush the lines and install a new master cylinder and refill with new fluid again. I have checked rear brake adjustment and it is good and the parking brake holds well against the engine in reverse and drive. If anyone has any other ideas or suggestions, I am all ears. I would appreciate any guidance or ideas I can get. Thanks
 

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I haven't had a defective master cylinder cause soft/spongy brake pedal. When the MC is bad, it is firm, but slowly sinks to the floor.

Did you bench bleed the MC?
Did you install the brake calipers on the correct side?
Either of those can lead to air getting trapped in the system. Air compressing gives it the spongy feeling.

Did you at any point use a rubber hose in place of a steel line? Like if you ran a 10ft long rubber line to the back of the car instead of using the steel line, that'll make it feel like crap.
 
I haven't had a defective master cylinder cause soft/spongy brake pedal. When the MC is bad, it is firm, but slowly sinks to the floor.

Did you bench bleed the MC?
Did you install the brake calipers on the correct side?
Either of those can lead to air getting trapped in the system. Air compressing gives it the spongy feeling.

Did you at any point use a rubber hose in place of a steel line? Like if you ran a 10ft long rubber line to the back of the car instead of using the steel line, that'll make it feel like crap.
Thanks for the thought starters. I am sorry if I wasn't clear enough with what I am experiencing the pedal does bleed down if you hold it but it doesn't feel firm enough to me from the get go. Also, if I pump it a few time it builds some pedal feel but then will bleed down. As far as rubber line, I replaced all of the steel lines with the same length and bends steel lines and only replaced the factory two front caliper hoses and the one factory rear differential hose. Everything else in the system is steel and new and I flushed all of the steel lines before I installed them. The debris may be just some loose rubber fragments from inside the new hose as I did not flush them before install. The pedal may not be related to the debris at all, I just worried it might be.
 

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When you bleed, how do you do it?

Make sure the rod from the booster is set at the right length (too short would have the pedal go down with no effect) and a quick feel with a screw driver on the MC piston will tell you right away if it comes back without hesitation as it should and offers good resistance.
If you have no leaks, and sure you bled the master and lines (where you saw clearly there was no air after flushing few cc's for each wheels starting at rear passenger one), then yeah, you might have the master piston seals not able to build pressure for some reason.

Any difference at the pedal if engine runs?
 
When you bleed, how do you do it?

Make sure the rod from the booster is set at the right length (too short would have the pedal go down with no effect) and a quick feel with a screw driver on the MC piston will tell you right away if it comes back without hesitation as it should and offers good resistance.
If you have no leaks, and sure you bled the master and lines (where you saw clearly there was no air after flushing few cc's for each wheels starting at rear passenger one), then yeah, you might have the master piston seals not able to build pressure for some reason.

Any difference at the pedal if engine runs?
Thanks for the suggestions, I will check the booster rod. I have inspected for leaks and found none, and I am pretty sure no air was coming through the clear hose. I was using a manual vacuum bleeder at first and then borrowed a power bleeder from a friend. I really didn't feel any difference with the power assist when the engine was running.
 
Thanks for the thought starters. I am sorry if I wasn't clear enough with what I am experiencing the pedal does bleed down if you hold it but it doesn't feel firm enough to me from the get go. Also, if I pump it a few time it builds some pedal feel but then will bleed down. As far as rubber line, I replaced all of the steel lines with the same length and bends steel lines and only replaced the factory two front caliper hoses and the one factory rear differential hose. Everything else in the system is steel and new and I flushed all of the steel lines before I installed them. The debris may be just some loose rubber fragments from inside the new hose as I did not flush them before install. The pedal may not be related to the debris at all, I just worried it might be.
tpg71mach, Regardless of the brake issue, your chassis and undercarriage workmanship is excellent.
 
Sounds like bad MC to me, too. Just as a sanity check, are the caliper bleeders towards to upward end of each caliper? Also, is the proportioning valve (I think the proper name is combo valve) centered? I had trouble with one that was off center and wouldn't bleed properly. Vacuum bleeder should not have that issue.
 
If you can build pressure by pumping the pedal, then it slowly drops to the floor, your MC is bad.
Thank you, that's what I suspected but I am not by any means qualified to be a technician. I can bolt and unbolt parts but not the greatest with diagnosis. I really appreciate the help. Now I guess I need to order a new master cylinder and get to work swapping it out.:(
 
Sounds like bad MC to me, too. Just as a sanity check, are the caliper bleeders towards to upward end of each caliper? Also, is the proportioning valve (I think the proper name is combo valve) centered? I had trouble with one that was off center and wouldn't bleed properly. Vacuum bleeder should not have that issue.
Thanks, yes the combo valve is centered and the front calipers are mounted with the bleeder heading up. I appreciate the suggestions, I will post once I get the new master installed and the system bled. Hopefully it will be good news.🤞Thanks all!
 
It’s been long time since I did that but there is if I recall a lock ring on the piston bore. You could inspect the piston/orings. Your local parts shop may have all you need at lower cost. You could also loop the lines with a temp one into the cylinder reservoir to see if the liquid is pumped normally like during a mc bench bleeding if you have some fittings/lines laying around…
 
You could also loop the lines with a temp one into the cylinder reservoir to see if the liquid is pumped normally like during a mc bench bleeding if you have some fittings/lines laying around…
They sell a kit for this at the parts store.
1692737479756.png


With him saying the pedal slowly sinks to the floor, that just about has to be the master cylinder leaking internally. Unless he's got a leak somewhere and a puddle of fluid under the car.
 
They sell a kit for this at the parts store.
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With him saying the pedal slowly sinks to the floor, that just about has to be the master cylinder leaking internally. Unless he's got a leak somewhere and a puddle of fluid under the car.
I ordered one of those kits with my new master cylinder. Thanks
 
@giantpune "With him saying the pedal slowly sinks to the floor, that just about has to be the master cylinder leaking internally. "

Yes I think so too... I mentioned the booster because his been rebuild. I had a similar case when I've replaced my 73 booster 2 or 3 years ago, by a smaller one from Leed. The rod was set too far away. and while somehow the pedal was not really going down on its own, prolly due to the membrane, breaking power was near zero. Basically I was barely pushing the piston, and when I was, the pedal was not having enough travel left to push more.

"They sell a kit for this at the parts store."
I thought so... you guys are so spoiled!! :D
 
Thanks, yes the combo valve is centered and the front calipers are mounted with the bleeder heading up. I appreciate the suggestions, I will post once I get the new master installed and the system bled. Hopefully it will be good news.🤞Thanks all!
Something I did not see reading through, but make sure your new MC has the correct diam piston for power brakes.
On my PB rod length, I set it with about .010" free play between the MC piston and the rod. I do this with a small piece of plasticine as a thickness guide. Drop the plasticine in the MC then assemble to the booster, snugging the bolts. Then remove the MC. the plasticine ought to stick to the rod. If the rod has pushed through, then it's too tight. Try again until you get a thin layer over the rod tip. The rod ought to be just contacting, but a little free play (.010") is better than too tight. Get the idea?
 
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Something I did not see reading through, but make sure your new MC has the correct diam piston for power brakes.
On my PB rod length, I set it with about .010" free play between the MC piston and the rod. I do this with a small piece of plasticine as a thickness guide. Drop the plasticine in the MC then assemble to the booster, snugging the bolts. Then remove the MC. the plasticine ought to stick to the rod. If the rod has pushed through, then it's too tight. Try again until you get a thin layer over the rod tip. The rod ought to be just contacting, but a little free play (.010") is better than too tight. Get the idea?
Thank you, I will check the free play when I go to install the new master cylinder. Also I am going to measure the bore when I receive the new one and the bore on the one I am removing. I did not check the first one before I installed it, so I will not make that mistake again. :D
 
Thanks, yes the combo valve is centered and the front calipers are mounted with the bleeder heading up. I appreciate the suggestions, I will post once I get the new master installed and the system bled. Hopefully it will be good news.🤞Thanks all!

The bleeder should point to the REAR of the car. If they're pointing up, you'll never get the air out - first hand experience showed me that.

I would swap the calipers side to side, re-bleed and see if you still have the sinking pedal.

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The bleeder should point to the REAR of the car. If they're pointing up, you'll never get the air out - first hand experience showed me that.

I would swap the calipers side to side, re-bleed and see if you still have the sinking pedal.

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Okay, thank you. I just checked my pictures and guess what? I screwed up and put the calipers on the wrong side, Duh. I guess I played a little too much football without a helmet when I was a kid, I have some dain bramage. I will swap the calipers and rebleed and recheck. I can always return the master cylinder if this solves it. That would also explain why the rear brakes are holding against torque in drive and reverse. I really appreciate your help.
 

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I guess I played a little too much football without a helmet when I was a kid, I have some dain bramage.
I guess!! BUT if you played a man's game, RUGBY, you wouldn't need to helmet... or padding, just a jock strap and some tape around your ears if you were a forward in the scrum.
Oh, but wait, women now also play Rugby and believe me, those are NOT women to mess with.
Anyway, glad you're making progress on the brakes. I have got to be honest, that's an easy mistake to make, but not hard to fix, just time and more brake fluid. Personally I use DOT 4 in my system with disc brakes.
 

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