HELP---need Brake EXPERT!!!

[Huck]

Tired of throwing parts at my Blazer brake problem. When you hit the brakes, the pedal drops half way to the floor before it stops. The tires squeel but I cna't tell if the front or rear are engaging. this thing supposely has a rebuilt master cylinder and I put another proportioning valve in there that is used but is supposed to work. I replaced the rear wheel cylinders, drum shoes, resurfaced drums and bleed them multiple times. I checked the front rotars, had them resurfaced and bleed them as well. Would really like this sob to stop better then a 61 falcon. Need someone who really knows brakes on these trucks and can walk me through the process of what to do first etc. I've looked at the books and they haven't helped me yet. The freakin brake light is one no matter what i do with the proportioning front pin thingy!!!!
HELP

[Gordo]

Huckster!
I would question the condition of that "rebuilt" master especially if your light is staying on. When you bled the system did you get a good "squirt" of fluid from each front and rear wheel or did it just dribble out the rear ones?
Another thought, that master someone replaced may not be correct one and your push pin sits deeper into the back of the piston than it should.

[crazy longhorn]

Ugg! I am not an expert by any means(or I would have big bucks!)Just a thought or 2....if the brake failure lite is on, you might look at that "so called" rebuilt master. Another prob that can cause that would be a leak in the hydrolic system....not a big one, but enough to cause a problem. I have chased that one a few times. the kids ranger.....new everything on the front, had a banjo fitting that did not seal(the hose bottomed on the caliper, due to chitty machine work), & needed an extra copper washer to seal. Thats not the only one I saw with that problem....if the parts you are tossin at it dont cure the mess, look for a small leak in the system.....not big enough to make a puddle, but a leak is a leak.....it still "slurps air" best of luck,crazyL

[franko72]

Maybe you need to bleed the air from the master cylinder first.Remove the brake lines from the master cylinder and get a cheap bleeder kit from your local parts store.Place the hoses in the brake line master cylinder openings and have them return to the reservoir with the cap removed.pump the brake pedal slowly ,each time you press the pedal wait about 5 seconds before you press it again. keep doing this until there are not any air bubbles in the reservoir. you can also tap the bottom of the master cylinder with a mallet as you do this to help release air.reconnect the lines and bleed the connections at the master cylinder the same way you bleed the brakes.Then go to the right rear wheel cylinder first and bleed it, then the left rear. next the right front, then the left front.sounds like too much air in the system or your master cylinder is bad.Master cylinders are less than 20 bucks here if you dont have any luck buy another

[cheyenne10]

Also watch any rubber lines. They can have weak spots in them that buldge when under pressure. Receipe for later disaster.

[walker]

Brand new M/C is about $15. Pre bleed the M/C before hooking up the brake lines. Use a vaccum pump brake bleeder, $29, and you will get excellent results.

[jlmtrout]

You might want to check your brake booster, vaccuum line and check valve. Also, if you have an incorrect proportioning valve, that could throw your brake balance off. Go ahead and spring for the new stuff. When you're talking about brakes, you can't be too careful. Also, the left rear is the farthest from the master cylinder when you trace the lines down, so start there first when bleeding the rear. Good luck and keep on truckin'.....Jerry

[Grim Reaper]

If that light is coming on you still have air in the lines some where or somethign else is wrong like your drum brakes are not adjusted up enough.

Now I'm real good at brakes on 73-87 but it's a little different on 72 and older. On the 73 up trucks gettign the Combination valve (what most people mistakenly call a Proportiong valve)bleed is a problem. The inlets are on the top and the outlets are on the bottom. So air stays at the top in a cavity around the proportioning valve side of the combination valve.

The trick I have found to get this air out is to crack open the inlets a couple turns till it starts leaking and tap on the combination valve (CV) to dislodge the air. The line in will gravity bleed so air should not get in it. Now this works on a 73 up fine but as I recall the 72 down the CV is mounted to the side of the master. For this to work the fluid in the reservior needs to be higher then the CV.

Now a little education. When the system is properly bleed and you apply the brakes there is a valve in the CV for the front disc's that holds off applying the the front brakes till some presure develops in the rear circuit. It holds off between 50-100psi. The Pin on the CV is bypassing that feature. That pin is misunderstood. You don't push it. You push around it till it's exposed and then you pull on the pin. There is a actual tool to do this. The pin has a ridge that is exposed when you push the end down and the tool hooks into that. Once that pin in the center is pulled then the front brakes will gravity bleed.

Lesson 2:

The double piston master cylinder only mechaincly drives the rear circuit when bleed properly. The pressure developed infront of that piston drives the piston forward for the front circuit. The pressure in the front circuit helps to ballance this by giving resistance to the rear circuit and then forces the fluid down the lines to the wheels.

If the rear circuit has no pressure then the rear piston will bottom on the front piston but it is excessive travel like your experiancing. Front circuit having air in it will do the same. The frotn piston will bottom at the end of the bore. Having the light on AND the excess travel confirms that one of the circuits is not building pressure properly.

That said if the rear drums on a disc drum systme are not adjusted up enough you get excessive peddle travel as well and have the same syptoms.

Start out by adjusting up the rear drums till they drag some. You should feel a slight drag and possibly hear it as well.

As for bleeding...the fartherest first is partly true for the wrong reasons. You want to bleed the first circuit in the master first. In this case it's the rear. Side does not matter because both rear lines are centerally fed at the pumpkin. on my 75 if you measured line lenght the driverside has about 2 inches more line so technically it's the farthest from the master and with should be bleed first according to that myth.

Now on the front. You want to bleed the side that has the highest line out of the CV if both lines go directly do it first. WHY? So you don't chase you own tail. Air rises in fluid.On my 75 the drivers front comes in at the top of the CV. If you bleed the highest line on the CV first you removing the most air.

If it's a single line out of the CV on the 72 and older then a T then the farthest would be the best.

Last but not least is a trick my pro wrench buddy says always works. Vacuum

On cars that he has a hard time getting to bleed he will find a reservior cap and install fittings into it. He then will pull a bout 3-5hg on the system for a few hours. This will pull the air back up to the reservior. You want ot be really careful not to pull a lot of vac or you can pull air in the seals. so light vac and time for it to do it's thing.

Another thing to check. There is some adjustment between the peddle and the master. On some vehicles is on the linkage from the peddle. On others you adjust the rod end that is exposed if you unbolt the master from the booster. On my 75 I know it's in the booster. Not sure on your older truck. When you see the peddle travel stuff in the repair books this is what your adjusting.

[67chevemall]

Nice Grim!

[68C15]

Grim is the brake Guru. take his word as gospel. one thing that works real good for 2 man bleeding is to open bleeder, push pedal, close bleeder, release pedal, wait 5 seconds & repeat. does a better job than pumping & opening bleeder. high volume-low pressure flows air out instead of trying to push it into inescapable places.

[Grim Reaper]

Originally Posted by 68C15

Grim is the brake Guru. take his word as gospel. one thing that works real good for 2 man bleeding is to open bleeder, push pedal, close bleeder, release pedal, wait 5 seconds & repeat. does a better job than pumping & opening bleeder. high volume-low pressure flows air out instead of trying to push it into inescapable places.

Your right about compressing it. I always have my pump person just lightly hold the peddle instead of trying to jam it through the floor. The seconday concern is you can bottom out the master and dmage seals so I never want my pump person to push particulary hard. Also with a old master their might be dirt, rust or trash in the master and if you run it past its normal operating range it will run over it and cut the seals. This will cause an internal leak that's REAL hard to diagnose.

Yeah I'm usualy doing this stuff alone so I use a mighty Vac. Once you pull the pin in the CV it works pretty good. My olderst daughter is big enough to pump the brakes but shes off at a friends or I'm doing late night wrench fests and she's already in bed.

[toddtheodd]

I agree with Grim.
I too had this problem. I tried bleeding the brakes but it didn't help. Pulled off the drums and adjusted them up nice and tight. No more problems. Turns out that the drums were way out of adjustment.
I have found that brake-torqueing causes them to go out of adjustment.