Loss of brakes & brake fluid in the cab..

[stock75]

Hello all, hopin someone can help me out, was drivin home frolm work today and lost brake prssure, pedal goes to the floor at which point the rear wheels will locl up. I noticed a small amount of brake fluid inside the cab underneath the break pedal, is there some sprt of seal here or what? My brake fluid was shot, id been planning on going through the braking system in the near future, guess the time is now. What do you guys think? Could the master cylindar have failed?
Thanks
Posted via Mobile Device

[gchemist]

Sounds like your master cylinder has failed.

[stock75]

So went and picked up a new master cylindar from o reileys, got the old one off only to find that the new one is different.. it is made by brake best but the indent that the brake rod sets into is not farr enough into the master cylindar so it will not bolt up, seems like the rod needs to be a half inch shorter.. do I cut the rod? I ordered a different brand from oreileys but it won't be here til the morning, I forget who makes it but the description says it comes with a new rod which is what makes me think I could possibly just cut mine and use the master cylindar I bought.. I'm at a loss and I workk in a few hours, looks like I'm taking the chevrolegs.
Posted via Mobile Device

[ray_mcavoy]

Your truck has manual brakes, correct?

It sounds like the new master cylinder they sold you is for a power brake setup ... those often had the shallow indentation (instead of a hole) in the back end of the piston.

Don't cut your pushrod! The pushrod sticking into that deep hole in the back of the original master cylinder piston is what keeps the pushrod in place. A shorter rod would just fall out of that little indentation in the back of a power brake master cylinder leaving you with absolutely no brakes.

[stock75]

Well I got the new master cylindar on, while bleedong the brakes I noticed the front brakes are not getting much pressure, only a bit of old fluid would trickle out when I cracked the bleeders, what's the surefire way to tell if the new master is workin properly? Should I crack both thr front bleeders andpush the pedal then see what happens? The rear brakes are working fine, the pedal seems stiff and if a slam on the pedal the rear brakes lock up.
Posted via Mobile Device

[cmyc5575]

Originally Posted by stock75

Well I got the new master cylindar on, while bleedong the brakes I noticed the front brakes are not getting much pressure, only a bit of old fluid would trickle out when I cracked the bleeders, what's the surefire way to tell if the new master is workin properly? Should I crack both thr front bleeders andpush the pedal then see what happens? The rear brakes are working fine, the pedal seems stiff and if a slam on the pedal the rear brakes lock up.
Posted via Mobile Device

Do you have a proportioning valve?

[stock75]

Not sure, truck is completely unmolested factory so if I'm supposed to I imagine I do, where is it located?
Posted via Mobile Device

[stock75]

Just refered to my repair manual and can't seem to find the proportion valve, are you mahybe refering to the combination valve?? The manual mentions combination valve a couple times, at one point saying it needs to be held at an open position when bleeding the brakes, what do they mean by this?
Posted via Mobile Device

[cmyc5575]

Originally Posted by stock75

Just refered to my repair manual and can't seem to find the proportion valve, are you mahybe refering to the combination valve?? The manual mentions combination valve a couple times, at one point saying it needs to be held at an open position when bleeding the brakes, what do they mean by this?
Posted via Mobile Device

Well the proportiong valve is usually a regtangular box looking thing up underneath the master cyclinder. This valve can be adjusted to control the amount of brake fluid that is getting to the brakes from the back to front, therefor you can have more brake force in the back then you would in the front.

[bk2life]

make sure you have the lines going into the front and the rear correctly. i know this sounds goofy, and sometimes there are different sized fittings, but double check.

[stock75]

[QUO TE=bk2life;5759226]make sure you have the lines going into the front and the rear correctly. i know this sounds goofy, and sometimes there are different sized fittings, but double check.[/QUOTE]

Great idea, so if I'm looking at the master cylindar, the brake line closest tome should be which one?
Posted via Mobile Device

[ray_mcavoy]

Originally Posted by stock75

Just refered to my repair manual and can't seem to find the proportion valve, are you mahybe refering to the combination valve?? The manual mentions combination valve a couple times, at one point saying it needs to be held at an open position when bleeding the brakes, what do they mean by this?
Posted via Mobile Device

As you read in the manual, these trucks use a combination valve. It has a proportioning valve, a metering valve, and a pressure warning switch all combined into one unit. On 73-87 trucks it's located down on the frame crossmember below the radiator support.

The proportioning valve section of the combination valve is designed to reduce pressure to the rear brakes to help keep them from locking up. The factory valve is non-adjustable.

The metering valve section of the combination valve is designed to momentarily hold off pressure to the front brakes to help with braking stability. This is the part that your manual is talking about holding in the open position while bleeding. There will be a little stem sticking out through one end of the valve (often covered with a protective rubber boot) ... holding that stem in keeps the valve open. However, holding this valve open for bleeding is usually only necessary when using certain pressure bleeder tools that don't develop enough pressure in the system to push the valve open on its own.

You did bench bleed the new master cylinder before installing it, correct? And what method are you using to bleed the brakes? I've found that the following method seems to work best if you don't have a pressure or vacuum bleeder tool but do have an assistant to help out:

(1) Check fluid level in master cylinder reservoir and top up if necessary, replace lid.
(2) Loosen bleeder screw.
(3) Have your assistant slowly push the brake pedal down and hold it there.
(4) Re-tighten bleeder screw.
(5) Have your assistant release the brake pedal.
(6) Repeat until a steady stream of brake fluid flows out of the bleeder screw as the pedal is being pushed down.