Brake Pedal Stiffens Until I Start Bleeding the Front Brakes

[Jimbo_74_Duster]

Hey All,

(Working on a 1974 Plymouth Duster 360, front end rebuild/drum to disc manual brake conversion)

I cannot get my brake pedal to stiffen up and I've bled the brakes 6 times now. What happens, is that it starts to stiffen up until I move to the front right brake. Once I bleed that, the stiffness in the brake pedal becomes almost non existent and it's all loose again. Here is the kit I installed. Here is the prop valve from Doctor Diff and here is the master cylinder.

(I am bleeding them by the book with someone on the pedal, correct fluid and in the correct order; rear right first etc. I triple checked my installation job of the proportioning valve and the brake line mapping and cross checked it with my hard copy shop manual. There is also ABSOLUTELY no fluid leaks on the floor. I'm guessing people are going to say the kit I bought is not a reliable one and or the master cylinder.

Thanks for the input

 

[Dana67Dart]

From what I see in your kit and MC and valve your components are golden, replicating 73 up system.

the only thing that might be is the MC is 1 1/32" which means the pedal will require a tad bit more leg pressure than other MC with smaller bores.

as for bleeding and soft pedal...

Did you properly bench bleed the MC BEFORE starting the in car bleeding?
and the MC tube routing is as in this FSM page from Dr. Diff?

the stiffness in the brake pedal becomes almost non existent and it's all loose again

define "Loose"

 

[Inertia]

Caliper bleeders at the top ?

 

[Jimbo_74_Duster]

From what I see in your kit and MC and valve your components are golden, replicating 73 up system.

the only thing that might be is the MC is 1 1/32" which means the pedal will require a tad bit more leg pressure than other MC with smaller bores.

as for bleeding and soft pedal...

Did you properly bench bleed the MC BEFORE starting the in car bleeding?
and the MC tube routing is as in this FSM page from Dr. Diff?
View attachment 1716466498


define "Loose"

I did bench bleed the MC before installing. But I am going t do it again and start over from there. I thought maybe it has to do with the disc brakes being new? So they......I don't know, need more bleeding or something but I've done it so much now and even continue after it's clear of bubbles for multiple extra pumps. I'll triple check the routing. Thank you. Maybe it's in the MC and needs an extra bench bleed.

Oh and regarding the 'loose" feel. Before the conversion the brake pedal was very stiff and only would depress a tiny bit before becoming very stiff. Now the best I've gotten it, is pretty stiff, but it sinks more the longer I hold pressure on the pedal, which is air correct? But once I go to bleeding the front brakes, it looses so much ground and doesn't stiffen up until the pedals almost to the floor and then is twice as spongy. So weird

 

[Jimbo_74_Duster]

Caliper bleeders at the top ?

Yeah I meant to add that, they definitely are at the top. Thanks

 

[Inertia]

You won't get good pedal until the pads are in full contact with the rotors.
If the rear brake shoe adjusters are too loose, can cause low pedal too.
Pull on the emergency brake to move the shoes into contact with the drums.
Pump till all pads in contact, rebleed .

 

[Dana67Dart]

Oh and regarding the 'loose" feel. Before the conversion the brake pedal was very stiff and only would depress a tiny bit before becoming very stiff. Now the best I've gotten it, is pretty stiff, but it sinks more the longer I hold pressure on the pedal, which is air correct? But once I go to bleeding the front brakes, it looses so much ground and doesn't stiffen up until the pedals almost to the floor and then is twice as spongy. So weird

sinking while holding the pedal and no fluid loss sounds like internal leak in the master. Also check under the dust boot on the inside of the firewall for signs of fluid.

 

[Killer6]

Yeah I meant to add that, they definitely are at the top. Thanks

There's a real possibility the differential valve moved over during pumping, and it gets close to centering, but as soon as You crack the front bleeder....it moves off again. Best method is crack the bleeders & let the fluid run on it's own, give the wheel cylinders & calipers a tippy-tap now & then w/a plastic mallet, wait for a decently bubble-free dribble out of each. Keep the master full of fluid during this, if fluid is not coming out of a bleeder, You can have the assistant do easy 1/2 pumps after You've closed the bleeders that were flowing, while You hold a finger over the bleeder just tight enough to prevent air getting sucked in on the up-stroke.
If the differential valve is stuck, You can open the bleeders that pump-up, then w/the other end bleeders closed slam the pedal....if it was stuck it will give a definitive POP when it re-centers.
I never bench-bleed masters anymore, leave lines loose & let them run as well, then when a decent amount of fluid is trickling snug them. Do the wheels etc., then go back & crack/bleed the lines at the master last.

 

[Trojmn]

A quick but sometimes messy sanity check to make sure the calipers and lines are 100% bled out is to pull ONE caliper off the rotor and pump piston a good way out while it is bleeder down/hose up. Tap it a few times then steadily push that fluid back up to the master. re-assemble and do the same thing to the other side. That large volume of moving fluid does wonders to push all the bubbles up and out.

 

[Bewy]

Agree, slowly sinking pedal could be an internal leak in the m/c.

 

[Dana67Dart]

There's a real possibility the differential valve moved over during pumping, and it gets close to centering, but as soon as You crack the front bleeder....it moves off again

the valve self centers with springs (see the cutaway) UNLESS it is dirty or poorly manufactured

 

[dano]

Stupid question, but are you using brake fluid? I know a guy who used power steering fluid once by accident, didn't look at the bottle.

Also, leaks do not necessarily drip. Some just wet the fittings.

Have you bleed the rear? Drums still I assume?

 

[Killer6]

the valve self centers with springs (see the cutaway) UNLESS it is dirty or poorly manufactured

Uhmmm, not when they're tripped, no they don't,...as long as there's a pressure imbalance and insufficient counter imbalance has been applied. I've bled thousands & thousands of brakes over 45yrs , if they're set hard by someone pumping the brakes like an angry monkey on crack, they will not just come back by getting the brakes bled.....yes, old.../dirty/crappy quality makes this easier to occur.....but can happen just as easy on a new one.

 

[Dana67Dart]

Uhmmm, not when they're tripped, no they don't,...as long as there's a pressure imbalance and insufficient counter imbalance has been applied. I've bled thousands & thousands of brakes over 45yrs , if they're set hard by someone pumping the brakes like an angry monkey on crack, they will not just come back by getting the brakes bled.....yes, old.../dirty/crappy quality makes this easier to occur.....but can happen just as easy on a new one.

we will have to agree to disagree.

it has not been my experience with any car I have owned Ford, Chevy or Chrysler

 

[92b]

Get some plugs so you can isolate your problem. Start by cutting the system in half at the master to determine if the problem is front or rear. Sounds like your problem is front but I wouldn't take that for granted. Plug the front off and see if you have good brakes. Next plug the rear and see if your problem reoccurs. If it does try plugging the rf caliper off and see if that makes the problem go away. And so on. Remember to rebleed wherever you put plugs.

 

[Jimbo_74_Duster]

Get some plugs so you can isolate your problem. Start by cutting the system in half at the master to determine if the problem is front or rear. Sounds like your problem is front but I wouldn't take that for granted. Plug the front off and see if you have good brakes. Next plug the rear and see if your problem reoccurs. If it does try plugging the rf caliper off and see if that makes the problem go away. And so on. Remember to rebleed wherever you put plugs.

Ok cool, this is a good. I’ll try that out Friday when I have time, along with all this other input from the good peeps here. Thank you

 

[Jimbo_74_Duster]

Stupid question, but are you using brake fluid? I know a guy who used power steering fluid once by accident, didn't look at the bottle.

Also, leaks do not necessarily drip. Some just wet the fittings.

Have you bleed the rear? Drums still I assume?

Not a stupid question, I definitely have done dumber stuff in the past. I am using the right fluid tho. Going to give it another go on Friday and start from the MC. Thank you

And yes they’re still drums in back

 

[Jimbo_74_Duster]

Thanks for the input everyone, I’m going to revisit this on Friday and will post up what I find after trying out this good stuff.

Thanks for the help. Keep it coming if you have more ideas, reading back through the manual now

 

[Inertia]

Uhmmm, not when they're tripped, no they don't,...as long as there's a pressure imbalance and insufficient counter imbalance has been applied. I've bled thousands & thousands of brakes over 45yrs , if they're set hard by someone pumping the brakes like an angry monkey on crack, they will not just come back by getting the brakes bled.....yes, old.../dirty/crappy quality makes this easier to occur.....but can happen just as easy on a new one.

we will have to agree to disagree.

it has not been my experience with any car I have owned Ford, Chevy or Chrysler

You're both sorta right .

Ferd and chubbie imbalance switches often needed to be re-centered when tripped.
Always a pain in the *** when installing line-lock on them, as the switch would trip and need centering every time you used the l/lock.
Most times you'd find the sensor wire pulled off.
Mopars were self centering except on the bigger trucks/mohos.
Springs center the shuttle when the pedal is released.
It's also why the Mopar "imbalance" lite comes on with the line-lock set, then goes off when line-lock released.
See pic

 

[Killer6]

we will have to agree to disagree.

it has not been my experience with any car I have owned Ford, Chevy or Chrysler

There's nothing to disagree about, My experience in My lifelong career has proven what I'm stating to be a fact, and has been necessary a few dozen times over those decades. Most often on GM's, but a handful of times on Mopars as well. "IF,...IT SHOULDN'T,....NEVER HAPPENED TO ME...."....irrelevant. I'm making the OP aware it's possible, because it is, that is all.
BTW, I thought the same way as You at 26 years old, and when such a car came in I told the tech trying to bleed it if they just stood on the pedal & had sumbody pop the bleeders open quick it "should" re-center. I stated something else was wrong, & when the Son of the Father/Son Owners suggested what I am, I scoffed.
Well,...I got schooled & humbled, & 'POP'(actually pop & boing as the lines resonate),.....it worked & I never forgot it. And over the years the lesson is clear, gravity bleed & patient, slow half-pumps prevents 95% of bleeding debacles.
Peace, Jim K6

 

[Killer6]

Thanks for the input everyone, I’m going to revisit this on Friday and will post up what I find after trying out this good stuff.

Thanks for the help. Keep it coming if you have more ideas, reading back through the manual now

Sadly, You may find a new part defective as well, it happens. I was installing new wheel cylinders on the back of a '78 TransAm belonging to the Store Mgr. of the local Advance Auto, Who of course provided His own parts, ......every time I thought I had it bled, it would get a bit soft again, & every time I'd get air bubbles out if the Pass. side cyl. again. Turns out I had Him get another,...defective assy., it would slowly let air suck in, as if the spreader cups/spring weren't keeping the lips on the seals tight...it did not leak. New unit & problem solved, never took it apart to see what was up, He just took it back. That was ~25yrs. ago, so no, it's not just 'todays parts'....it happens.
Good luck & keep Us updated..

 

[Jimbo_74_Duster]

Got the brake pedal very stiff after re-bleeding the master cylinder much more and then redoing the brake bleed. BUT, now I found a leak at the prop valve with the front right (old brake line)

I knew I should have just re did all the lines when I started this. Going to re run new lines now just to go 100%

I did re run the rear brake line so that’s done. But for the front lines I figured I’d remove the prop valve, and attach the lines to it first, then drop it in from above to mount it and finesse the lines into place.

Otherwise the tight space is very difficult with the prop valve mounted, to connect lines to it.

Any advice before I start? The physical part of running the lines seems a little tricky

Thanks

 

[Killer6]

Got the brake pedal very stiff after re-bleeding the master cylinder much more and then redoing the brake bleed. BUT, now I found a leak at the prop valve with the front right (old brake line)

I knew I should have just re did all the lines when I started this. Going to re run new lines now just to go 100%

I did re run the rear brake line so that’s done. But for the front lines I figured I’d remove the prop valve, and attach the lines to it first, then drop it in from above to mount it and finesse the lines into place.

Otherwise the tight space is very difficult with the prop valve mounted, to connect lines to it.

Any advice before I start? The physical part of running the lines seems a little tricky

Thanks

I'd put whichever is hardest to start on & leave the other off, then put them in place, then get the other fitting started while You can still move them around. Trying to attach both & install together will suck.

 

[Jimbo_74_Duster]

I'd put whichever is hardest to start on & leave the other off, then put them in place, then get the other fitting started while You can still move them around. Trying to attach both & install together will suck.

So I cleaned all of the connections and re-tightened. I then pressed the brakes for ten minutes in and out and holding and they’re all bone dry now. Going to check again after a few days to double check but hoping it was just a loose connection

 

[Frank Mopar]

So the safety switch would ground and turn on the warning light I assume. So a test light connected between pos and the switch, the test light would light if it was grounded.