• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Diagnosing Braking Problem

xDread92x

xDread92x

Member
Joined
Feb 5, 2015
Location
NC
Brake Failure light has been coming on. I replaced a caliper and new brakes all around. As well as the master cylinder. I bled all the calipers from furthest caliper to closest to the cylinder. Brakes feel very firm until I turn the car on. That's when I lose pressure.

If I pump several times the pedal will firm up. But I notice when I press the brake the idle in the car drops. I hear the whoosh and see air entering the master cylinder reservoir. At times the pedal will sponge to the floor. I removed the boost check valve hose and tried to blow through the intake end and no air pushed through. I blew from the check valve side and air moved through 50% with the diaphragm inside sounding rough. I removed the valve and checked the hose for any areas where air could escape. Air blew strong through the hose.

Also when I removed the check valve from the booster a lot of pressure removed. So I do believe the brake booster is making proper pressure. At this point I am really leaning towards the brake booster check valve being bad.
 
I wouldn’t rule out the brake booster but what causes them to go bad? And how would I know? I know they don’t usually go bad
 
Mine failed instantly clearly identifying itself as the problem. The front cover fell off.

Sounds like you correctly identified the problem but with all the replacement and bleeding you might have extended the master cylinder beyond its wear area and caused sealing problems.
 
Last edited:
Crap I did not bleed the master cylinder! Would that cause my issue by chance? I figured it would bleed through the calipers thats why I didnt.
 
The MC wil ultimately bled through the wheel cylinders but only after a couple quarts of brake fluid. Best done first.

Only after all the air is out of the system will the brake light switch balance out in the off position.

The check valve is a very low cost item and probably should be changed after 30+ years anyway.
 
Can i still bleed the MC and see if anything changes? What would cause the brake booster to be bad?
 
Ok so I bled the master cylinder. There wasnt air bubbles but amost like some sort of semi bubble reaction through the lines. Im guessing this was just from the fluid and pressure. It did stop the idle drop when braking but i still see air going into the master cylinder when I brake. I have yet to rebleed the calipers.

I did some brake booster checks and it seems to pass. I kept my foot on the brake and turned the engine off. After 30 seconds of keeping my foot down on the pedal it firmed up nicely. The second test I did was pump the brakes with the engine off to remove vacuum. I then kept the brake pressed and started the engine. The pedal moved down towards the floor after starting. I also ran the engine afew minutes and cut it off. I pressed the brake 3 times slowly and each time the pedal got firmer.

My check valve is allowing suction from the intake. It feels strong enough. Should I bleed the calipers before continuing to diagnose? I just dont like the fact my pedal is firm when the car is off and as soon as I start it the pedal goes to the floor.
 
What year and model are we talking about, '92 240?

Which caliper did you change, front or rear? Did you install a reman or used caliper?
 
xDread92x said:
Ok so I bled the master cylinder. There wasnt air bubbles but amost like some sort of semi bubble reaction through the lines. Im guessing this was just from the fluid and pressure. It did stop the idle drop when braking but i still see air going into the master cylinder when I brake. I have yet to rebleed the calipers.

I did some brake booster checks and it seems to pass. I kept my foot on the brake and turned the engine off. After 30 seconds of keeping my foot down on the pedal it firmed up nicely. The second test I did was pump the brakes with the engine off to remove vacuum. I then kept the brake pressed and started the engine. The pedal moved down towards the floor after starting. I also ran the engine afew minutes and cut it off. I pressed the brake 3 times slowly and each time the pedal got firmer.

My check valve is allowing suction from the intake. It feels strong enough. Should I bleed the calipers before continuing to diagnose? I just dont like the fact my pedal is firm when the car is off and as soon as I start it the pedal goes to the floor.
Click to expand...

Your problem has nothing to do with the brake booster. You either have air trapped in the system, or, you have a damaged seal in the master cylinder on the piston. The 240 brake system is notorious for having air trapped in the lines.
 
Yea im going to bleed the calipers and see where that leads me. I feel confident I got the air out of the MC so ill see what happens. If bleeding the calipers doesn’t help I will rebleed the MC some more. It was a rebuilt MC so it shouldn’t be the MC itself or the seals.
 
Did you have someone help pump up the brakes and then while holding the pedal down, crack the lines loose at the master cylinder for a split second? The air could be trapped just past the master cylinder in the first part of the brake line. I have run into that countless times with 240s.
 
Bled calipers and still same deal. I noticed there isnt as much air going into the MC though. I guess I need to keep bleeding the master cylinder i guess it takes a long time to get the air out. When i bled the calipers there was still large air bubbles coming out. I dont really know if I will get anywhere with this but i’ll see
 
So I just did 4 runs of the MC Connector bleed with someone holding the pedal down. Everytime I opened them lots of pressure came out with fluid. Still same amount of air in the MC. There has to be a way air is getting in somewhere. All caliper drains are closed tight.

I removed the rear passenger brake line to replace the caliper and im not seeing any loss of fluid in the area. I feel like if fluid could make its way through then so could air. I can double check the connection that meets at the axle again but I dont know where else air could be getting in. This is a super annoying problem
 
Hey guys I just came across something on the internet. So when I got my brake caliper from oreilly auto I put it on and noticed the bleeder valve was on the bottom. I figured this was just the way they made it.

On another forum a guy said he had this happen and went insane trying to figure out the problem. Apparently air gets stuck in the top of the caliper and with the bleeder on the bottom doesn’t help get the air out. I think this is my problem. They gave me a left instead of a right, someone messed up. I think this is my problem
 
Oh damn I didnt even see his response. My bad. Yea it has to be the problem but bs for sure. I had to wait 3 days for it to get here, then spent the last 2 days messing with it. And now have to wait 4 days for it to get here on Wednesday. IF they send the right one and don’t screw up again. But yea that makes total sense why i cant get the air out of the system.

You would think when bleeding the caliper air would come out regardless of top or bottom bleeder from the pressure but I guess thats not the case.
 
xDread92x said:
Oh damn I didnt even see his response. My bad. Yea it has to be the problem but bs for sure. I had to wait 3 days for it to get here, then spent the last 2 days messing with it. And now have to wait 4 days for it to get here on Wednesday. IF they send the right one and don?t screw up again. But yea that makes total sense why i cant get the air out of the system.

You would think when bleeding the caliper air would come out regardless of top or bottom bleeder from the pressure but I guess thats not the case.
Click to expand...

In a vertical drop the fluid just goes right past the air. That's why it is so hard to get it out of the lines right at the master cylinder. Understand now?
 
You must log in or register to reply here.
Back
Top