brake predicament

[possum70]

1978 blazer, 350/350/203
affectionately known as Moira

I changed brake calipers. Back brakes bled fine. Front brakes... not so much. There are no bubbles but they don't want to build any pressure at all it seems. Pumping them doesn't do anything. To the floor every time. I checked the proportioning valve and know where the reset is but it doesn't push in. I don't wanna push in to hard but I'm a little concerned. I kinda need my brakes, you know?

possum

 

[4xcrazy]

sounds like a master cylinder issue.

 

[possum70]

But it was working before I changed the calipers. Can they go bad like that? I had changed the brakes. The calipers were rusty. The truck started pulling to the right pretty bad when braking so I figured calipers were inexpensive to replace, so I did. The brakes pulled but worked. I changed the calipers but now nothing. Could the MC go out that suddenly?

possum

 

[British Slave]

You still have air.

 

[Fordum]

When something like that happens, its almost always one of two things. Either an air bubble you have not found, or the MC.

In some vehicles, the left and right calipers are interchangeable, but the bleeder valve is on the bottom if you put them on the wrong side, and they cannot be bled.
I don't think your truck is one of these, but its only a second to check.

If no fluid is running out anywhere, then odds are its the MC.
This is actually a fairly common occurrence when brake work is done.

Disk brake MCs are real prone to this, due to the way a disk brake adjusts.
Since a disk brake always stays the same distance from the rotor, you never push a MC plunger any deeper as it wears.

As a result, it travels in the same part of the bore all the time. Which allows dirt, rust, or general roughness to build up in the part unused.
When you bleed the brakes, you suddenly push the plunger all the way to the bottom. If its been a while since that happened, then the plunger seals can get torn and ruined.
I don't find it unusual to have to replace a MC after a brake bleeding.

 

[Fordum]

Oops, I meant to add that I would suspect air before the MC even so.

There are several stories about building a pressure bleeder. You might want to consider one of those before you throw a new MC at it.

 

[British Slave]

I would try bleeding the MC and then bleed the brakes starting with the right rear, left rear, right front and finally left front.

 

[possum70]

would it be prudent to just replace the MC (15.99) and just do the entire system again? Would this keep the current MC from just crapping out totally? I mean, I can't figure out where anymore air could be and I'm not losing any anywhere?

possum

 

[Fordum]

Might be the best. If the old MC has lots of miles on it, the seals are probably worn due to the bleeding.
Even if its good, it may start giving problems down the road.

 

[possum70]

yeah, I guess that's next on the list. I really can't wait to get her started on the road again. I wanna see if the speedo works now.

 

[Fordum]

yeah, I guess that's next on the list. I really can't wait to get her started on the road again. I wanna see if the speedo works now.

Well, you don't need brakes to see if the speedo works........

 

[diesel4me]

I have used a hot roders trick to bleed brakes quick & easy,by getting a long peice of hose 3/16" I.D. and hook one end to the bleeder screw,and the other end to a vacuum source on a running engine ,you open the bleeder and leave it open until you hear the engine suck some brake fluid in,then close it--it wont hurt anything,just make some white smoke ..if you want to make a vaccuum bleeder ,all you need is a glass jar with a metal top,two peices of 3/16" brake line and enough clear vinyl tubing to reach all 4 wheels...you poke two holes in the jar lid,instert one peice of brake line so it is only 1" down into the jar,the other one goes almost to the bottom,and you solder or epoxy putty them into the lid...then put enough brake fluid in the jar to cover the tube closest to the bottom,and connect that tube to the bleeder,and the other tube to the vacuum source on the engine--I use vise grips as an "on-off" switch to control the vacuum ,its easy to see when no more bubbles are being pulled out of the brake system with this setup,and no pumping the pedal 100000 times,which can wreck the seals in the master cyl,as Fordum described--it sure beats trying to bleed them alone too,something I often must do..

You brake HOSES could be why it was pulling to one side also,old ones sometines have the inner lining of the hose delaminate and act as a one way valve,it'll let pressure go TO the caliper or wheel cylinder,but not let it release when you take your foot off the brakes...this could make bleeding difficult or impossible also...