What is going on with my brakes??!!!


Here are my symptoms:
-Spongy pedal all the time
-Brakes will usually stop the truck ok, but sometimes my foot goes right to the floor and the truck keeps on rolling; this happens maybe 1 in 10 times, but that is one time too many
-All brake lines and hoses are brand new, master cylinder is a new unit, not a reman
-Brake light on dash does not usually come on when my foot goes right to the floor
-No brake fluid leaks anywhere, reservior is always full
-Pedal "hisses" when depressed
-Drivers front bleed screw will usually hiss at me when opened, but the brakes have been bled 20 times, no exaggeration
-Master cylinder has been bench bled

This is really starting to bother me, as i cannot drive the truck in case it does not stop, and i am getting sick of throwing parts at it. I am thinking brake booster, but what is a sign that it is on its way out? ANy help is more than appreciated.

eamonn

 

Your brake fluid level does not go down... there are no leaks. If your front brakes hiss at you when being bled I would assume that air is getting into the system somehow. If it is, then your fluid level should rise. If it is not then the fluid may be leaking somewhere hard to spot. Look under the dash where the pushrod enters the master cyl. Is the pushrod the right length? Was some bit left out or not adjusted properly. In my experience this usually results in brakes getting harder and not releasing rather than getting softer. Years ago there were some mc's which could leak fluid into the intake via the brake booster. Don't know whether the vacuum pump on the cummins would let this happen. I hope someone with more helpful advice replies.

 

something worth having a look at....had a Merc with similar symptoms...check the front wheel bearings.......if worn/out of adjustment...brake disk ( rotor) can push the caliper pistons back...when you press brake...nothing the 1st time...usually works 2nd time pressed.......worth checking...

 

The hiss when the pedal is pressed is a leaky booster. That will not cause a low pedal, though.

What is your brake bleeding procedure?

The only thing you haven't mentioned is the dump valve for the RWAL system. That can cause a spongy pedal, but it does not explain air in the front brake lines.

 

Bleed precedure: PR, DR, PF, DF.....My RWAL has been bypassed because it was too rusty to get the bleeder screw or the line fittings out of. My line to the rear comes out of the splitter on the front of the frame back to the splitter hose above the rear axle.

The booster is the thing on the firewall behind the master cylinder, right? Napa has one for about $100, but i hate randomly throwing parts at a problem. Do my symptoms lead one to belive that a bad booster might be the problem?

--I have never had brake problems with anything i ever drove, so all this brake stuff is totally new to me. I used to be a truck/semi mechanic, so i can do air brakes all day, but hydraulic brakes is a whole 'nother world

 

My advice is to pull the new master cylinder and get one from a wrecker or some such cheapo place. Toss it in and see if your problem is still there. Trouble shoot by eliminating anything which has been changed recently... and anything which may have been accidently messed up when the work was done.

 

And replace the hissing vac. booster will you there!........It may not be all of your problems but its one.

 

Eamonn-

By bleeding procedure I mean how do you bleed the brakes, not what order.

The booster is not the cause of the dropping pedal - but it is what's making the hissing noise. Replace it, if it's hissing it is not working as well as it should be causing higher pedal effort.

 

bleed procedure: pump pump pump pedal till it is supposed to get hard (it never does) open bleed screw, let it **** out, close it. do it again if i hear. move on to next one. keep checking fluid level in MC so it does not go dry or even close.

I cannot say that my booster makes the pedal any harder to press. its actually quite easy, and requires less effort than my '93 1/2ton gasser. i could care less if my pedal is hard to press to be honest. its the fact that the brakes are always spongy at best, and the fact that my foot will just randomly go to the floor while trying to stop at a redlight or something. The thing taht gets me most is that there are NO fluid leaks anywhere, and the MC is always full.

-Side note: It usually takes my pedal at least half of its travel length to engage any sort of braking action from the truck. Possible overextention of wheel cylinders or something??

Thnaks for all the help from everyone, next rounds on me

 

did you have the calipers unhooked before this started?

Greg

 

you might have just answered or found you salution!

Try and fallow my theory......if your back brakes are so far out of adj(or something haywire in there) that the fluid going to the rear brakes "fouls" the brake porportioning into thinking that a line is broke in the rear and causes the brake fluid to be shut off causing the low brake peddle!
when you brake go two the floor is when the brake porportioning valve dont catch it in time? you wouldn't be loosing any fluid!?

Just a theory it may not hold water, but worth giveing you rear brakes a thorough inspections!!!!

Can you make any tires slide in a panic stop!

 

No, Why? Am i supposed to?

 

Replace your new master cyl. New means nothing. Its bad. The hissing is from all the travel of the pedal, it will be ok. There is a tear in one of your cups and it doesnt leak internally all the time until it gets into the right position. Adjust your rear brakes after replacing master and you will be all better. Also bench bleed the "new" new master cyl very good, sometimes small bubbles are tough to get out. -RATCHET-

 

nope...I was just trying to rule things out...

I spend most of my time working on Jeeps and the calipers will fit both sides...if you get them on the wrong side, they'll physically fit and seem to function, but the bleeder valve ends up on the low side of the caliper and you can't ever get all the air out...

Greg

 

I believe that you mentioned that your experience was with air brakes. Just in case, bleed hydraulics this way:

1. Stick a clear plastic line over the bleeder nipple, run the line into a bottle with some brake fluid in it.

2. Press and hold brake.


3. Have someone release the bleeder just enough to let the pedal go to the floor.... or if it is on the floor, spit out some air or fluid.

4. Close bleeder.

5. Release brake.

6. Check MC fluid level.

7. Repeat till fluid is clear and airless.

8. Go on to next wheel and repeat.

Sorry if this just repeats what you already know but I thought it might be necessary because of your airbrake experience.