inconsistant peddle

[ecw329]

ok so we did the MPB disc conversion on the duster and replaced everything, the problem is even after bench bleeding the master and countless bleeding of the brakes (rear right, rear left, front right, front left) the brake warning light keeps coming on intermitantly and sometimes will stay off until you push very hard on the peddle and it will come on again also, if you pump the brakes a few times then depress the peddle the light wont come on even if you push as hard as possible but when you let up and try it again the light will come back on after you depress the peddle say half way or so....

so is this a master cylinder problem as this is a rebuilt unit that we have had to change out bc the one in the kit leak through the real seals, we are at a loss to figure this one out because we are no experts anyways thanks in advance!

 

[camd64]

you said countless bleeding the brakes but are you getting air bubbles everytime you bleed it? also does the pedal height feel ok and is it always at the same or is the pedal sinking lower and lower?

 

[68gts340]

a couple simple things we offen over look , is the e- brake backed off ? did you adjust up the rear brakes?...
have you driven the car? ...if not you may just be overpowering the system by pushing to hard....if you push hard enough you can actually expand the rubber hoses......try driving it a little just to see how it stops...

 

[6pk2goDemon]

a couple simple things we offen over look , is the e- brake backed off ? did you adjust up the rear brakes?...
have you driven the car? ...if not you may just be overpowering the system by pushing to hard....if you push hard enough you can actually expand the rubber hoses......try driving it a little just to see how it stops...

Great info 68gts......

Pushing too far or low on the pedal when bleeding can actually blow the seals in the master. Bleed with a "soft" foot........not to the floor.

 

[RAMCHARGED]

if it's triggering the light it is usually the combo/proportioning valve that controls the dash light or the handbrake.If you let the peddle go to low when bleeding you can get the valve inside the prop/combo valve stuck to one side or the other.Try tapping the valve while someone pumps the brakes you may find it suddenly releases & then bleed again, but don't ever let the brake pedal get pushed all the way to the floor or it may stick again.I've seen this happen a few times & it's very difficult to diag.Also try & turn each wheel individually while someone holds the brakes & see if the brakes are actually holding.(drum brakes must be adjusted properly to get good pedal feel)You need to take your time & eliminate possible issues before throwing parts at it.I would also make sure that the m/c pushrod is the right length because if it is to long it will press the valve to far past the compensating port & give you line pressure problems & sometimes dragging brakes.Again take your time & make sure it's right & safe before risking you & your cars life.

 

[Longgone]

It sounds like air is still in the system giving you too much pedal travel. If you are not getting air out of the system with normal bleeding you can try this old method. Open or remove the master cylinder cap then go to each of your bleeders and JUST CRACK them open. Put something under the bleeders to catch the drips of fluid. Leave it overnight. Come back in the morning and close all of the bleeders and replace the mc cap. Try the brakes, if they`re adjusted right the pedal should be high and firm. Old timers swear by this method, personally I haven`t used it but it`s worth a try.

 

[fishy68]

It sounds like air is still in the system giving you too much pedal travel. If you are not getting air out of the system with normal bleeding you can try this old method. Open or remove the master cylinder cap then go to each of your bleeders and JUST CRACK them open. Put something under the bleeders to catch the drips of fluid. Leave it overnight. Come back in the morning and close all of the bleeders and replace the mc cap. Try the brakes, if they`re adjusted right the pedal should be high and firm. Old timers swear by this method, personally I haven`t used it but it`s worth a try.

I have done this many times on cars and forklifts and it always works good for me except one thing I'll note. Some drip slowly and some of them will empty a master cyl in 20 minutes so you have to watch the M/cyl to make sure it doesn't go dry.

The method of bleeding the brakes mentioned by the guys above is dead on. Never let the pedal go all the way to the floor or you'll never get all the air out. Also make sure the rear brakes are adjusted correctly like they said too. If their not adjusted right it'll give the feel like their is still air in the system.

One last thing I'll say is I hate to hear it's a rebuilt M/cyl. When I restored my car I went through 3 rebuilt units and they were all junk. One leaked, another wouldn't pump up like your saying yours is doing, the 3rd one I picked up and before I left the auto supply I took it out and looked it over good and found one of the brake line threads all buggered up and when I took the top off the surface was pitted up real bad and I'd bet it would have leaked out from around the cap. I got on NAPA's website www.napaonline.com and found I could get a brand new one for just $10 more than a rebuilt. Why they didn't tell me that I don't know. :axe: I ordered it and put it on and it has worked perfectly ever since.

 

[6pk2goDemon]

One last thing I'll say is I hate to hear it's a rebuilt M/cyl. quote]


100 % agree.....rebuilt m/c is not worth a sh^t if you can obtain a new one.

Lots of great info including to make sure the rear brakes are adjusted up correctly......It's where your pedal height comes from.

 

[NiceFishEh]

Sounds like your prop valve is working since its detecting a pressure problem.

Thanks to the fine folks here, I failed to ensure that the calipers were on the right side. The bleeder MUST be straight up or you won't get the air out.

I placed mine behind the spindles and got them backwards. Easily done. Was sure I had it right, but they were swaped side to side. Problem solved.

Hope this helps

 

[RedFish]

Are the calipers on the correct side of the car ? The bleeder screws need to be at the top of the fluid cavity. Simple error that is all too frequent.

 

[ecw329]

ok, to answer some of the questions... the calipers are on the right sides with the bleeder on the top, i agree about the rebuilt masters being crap as the first one had a huge leak in it allowing fluid to bypass the piston and leak onto the floorboards.... also i have driven the car on multiple occasions and when the switch isnt tripped it stops fine but when it becomes tripped you have to push the peddle past a noticable point of changing peddle pressure to get it to really stop.....also im thinking that due to the fact you can "pump up the brakes" it may be mc related..... o one mroe thing the MP brake kit came with a gm mc and that was the one with the huge leak we replaced with a stock a-body mc for disc/drum setup, is it possiblke the bore size on this unit is too small for the volume requirements? thanks for all the responses it took me a while to readem all!!!

 

[ecw329]

o about the bleeding.....it seems no matter how many times we bleed it air seems to come out so its as if its sucking in air from somewhere there are no system leaks, is this a symptom of a bad M/cly? tomorrow we will plug both outlets of the master and check for peddle sag and help diagnose a bad master

 

[68gts340]

a pedal that will pump is is a good sign of needing adjusting , but the changing paterns while driving the car and stopping it says master cylinder,, i would double check both....

 

[camd64]

sounds like the master but could also be a bad fitting somewhere. sometimes they are sneaky and leak air in and what seems like no fluid out.

 

[oldkoot]

GM master cylinders have the front brakes connected to the front bowl, rear brakes to the rear bowl-exactly opposite to a Mopar master cylinder. Did you reverse these lines when you changed master cylinders? Also there is a residual pressure valve built into the master cylinder for the rear drum brakes so if the lines are reversed the front disc's will be dragging and the pedal will be low. Too much pressure going to the rear drums and not enough pressure to the front discs could be causing the combination valve to go over center and turn on the dash light.Now a smaller bore piston in the MC will give you higher line pressure but there will be more pedal travel because of the reduced fluid volume. If the pedal is soft and spongy you have air in the system. You should have 900 plus psi going to the fronts and 600 to the rears. I just did the same conversion with the MBP kit that used a stock Mopar disc/drum mc and have 1100 psi at the calipers, pedal is firm and stops very well. Manifold vacuum is also important to how the booster works. You need 18 inches at idle to work properly

 

[ecw329]

o its a manual master cyl... today ran through all the fittings and made sure everything was tight think we might have found the trouble fitting sure were had to get to.... also the stock front disc master is reversed too, it even has casted into it "R" and "F" to help ya not screw it up!

 

[ecw329]

ok well we are having problems bleeding the back brakes, everytime we crack open the bleeders there the valve cuts fluid from the rears and diverts solely to the fronts how do we get past this dilemma? i guess the valve is doing its job....

 

[RedFish]

Wierd. If bleeding the rear brakes empties the front resivoir the lines at the valve must be reversed. Just a logical guess.

 

[oldkoot]

The combination valve has gone over center and has cut the fluid flow to the rear brakes-does this sound like the problem you are having? When you bleed the brakes DON'T PRESS THE PEDAL TO THE FLOOR WITH THE BLEEDERS OPEN. When the pressure drops on one side the spring inside the valve forces the leaking side closed.Turn the ignition key to the on position, the brake light should be on, now apply pressure to the brake pedal and the light should reset. If the light is still on jab the brakes quickly a number of times.