brake question
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Fleet Mechanic
Joined: Nov 2000
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From: In my House
brake question
...Is there anything special that needs to be done for brake bleeding after putting in a new master cylinder? I decided today to flush my system with my vacuum bleeder and now my pedal slowly falls to the floor at a light...No problems in a panic stop or otherwise, just falls to the floor slowly when braking...no leaks, so I'm assuming a master cylinder is an order....
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Fleet Mechanic
Joined: Nov 2000
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From: In my House
..SO the new master is in, and it was bench bled, and I bled all 4 corners and no bubbles are coming out, and the pedal STILL slowly bleeds to the floor!!No leaks anywhere.....What now?....Keep in mind, this all started when I decided to flush the system yesterday with my vacuum bleeder.....
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Fleet Mechanic
Joined: Nov 2000
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From: In my House
Nobody has had this problem before? Im just trying to figure out if the abs system plays a part in this bleed down. Some people say the abs system doesnt come into play when putting in a new master cylinder??
Fleet Mechanic
Joined: Jul 2010
Posts: 1,354
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From: Winnipeg
If you can't find any fluid leaks anywhere have you tried bleeding the system the standard way with someone pumping the pedal slowly and you cracking the bleed screw? A clear hose attached to bleeder and put in jar with some fluid to see if bubbles are still present.
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Fleet Mechanic
Joined: Nov 2000
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From: In my House
Thanks Lime for your input.....Yes I did it the vacuum method first (which seems to have gotten me in this mess to begin with!!) and then I replaced the master andhad the wife jump in the drivers seat. I went thru almost 2 big containers of DOT 3...Now I'm leaning toward the HCU....appprently the dump valve for the abs wears inside and allows the master cylinder to "dump" some of the pressure into the abs unit, bypassing the lines!! Time will tell.....will test out tommorrow....
Fleet Mechanic
Joined: Jul 2010
Posts: 1,354
Likes: 31
From: Winnipeg
Yes, I was just reading a thread on the HCU and it sounds like it may solve your problem. The valves may be contaminated by old dirty fluid, who knows?
I was told a long time ago with ABS systems, if your relining your brakes, always loosen the bleed screw on the caliper your pressing the piston back into to allow the dirty fluid to drain out instead of being forced back up the line. It prevents possibly pushing dirty fluid back up into the abs components. I'm also somewhat lucky driving on winter snow and ice the abs is always functioning and moving the fluid around in the system.
I was told a long time ago with ABS systems, if your relining your brakes, always loosen the bleed screw on the caliper your pressing the piston back into to allow the dirty fluid to drain out instead of being forced back up the line. It prevents possibly pushing dirty fluid back up into the abs components. I'm also somewhat lucky driving on winter snow and ice the abs is always functioning and moving the fluid around in the system.
Hotshot
Joined: Jul 2005
Posts: 11,931
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ABS keeps the brakes from locking up and sliding the tires, so it obviously has an "internal leak" to keep the pressure low so the brakes dont lockup.
Im not an expert on the system, but i have read several threads similar to yours where the conclusion was the "ABS" was the probable cause.
Im not an expert on the system, but i have read several threads similar to yours where the conclusion was the "ABS" was the probable cause.
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Fleet Mechanic
Joined: Nov 2000
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From: In my House
Update...so I ordered a "used" abs module for $150 and it should be in tommorrow, so I decided to take mine apart tonight. I took the motor off the module and found that it has a round piece on it that was drilled off cnter to "wobble" between 2 valves when it spins. Well one of the valves would not depress, so I decided to take the plug off the side of the block. It was a allen head and it quickly rounded out. I took my die grinder and made it into a flat head and gave it a few quick hits with a chisel and it started spinning out. I took the valve out and found that it was very stiff, so i cleaned it with some brake fluid and free'd it up to move again, so I reassembled everything and cleaned everything up and put it all back together and on the truck and I took it for a ride...guess what..I have no more sinking pedal!! I will test it out more tommorrow after work and bleed the system out better and report back.....regardless, I still will have the used one as a spare when it comes in....
fingers crossed!!
fingers crossed!!
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Fleet Mechanic
Joined: Nov 2000
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From: In my House
So I put the new abs module in and I'm still having the same issue
... Not as bad though. I can come down the street and slam on the brakes and it'll stop no problem, but coming to a traffic light, it'll slowly seep down, but if you release and then stab the pedal again...It'll hold with going down. It doesnt seem as though its as bad as before. I'm trying to figure out why I keep drawing air bubbles from my pass side rear caliper though....Is it possible for the caliper to be allowing air into the system? This all started last weekend when I had that wheel off to check the parking brake shoes....I did push the caliper back a little bit to get it on the rotor again...then proceded to flush the brakes with my vacuum bleeder.......any advice?? I'm open to all options...
... Not as bad though. I can come down the street and slam on the brakes and it'll stop no problem, but coming to a traffic light, it'll slowly seep down, but if you release and then stab the pedal again...It'll hold with going down. It doesnt seem as though its as bad as before. I'm trying to figure out why I keep drawing air bubbles from my pass side rear caliper though....Is it possible for the caliper to be allowing air into the system? This all started last weekend when I had that wheel off to check the parking brake shoes....I did push the caliper back a little bit to get it on the rotor again...then proceded to flush the brakes with my vacuum bleeder.......any advice?? I'm open to all options...
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Fleet Mechanic
Joined: Nov 2000
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From: In my House
....Issue is still there but not as bad...I have put almost 300 miles on it since then. I have also brought it out to the field to "activate" the abs...I heard the motor and felt it through the pedal...I was hoping to have that free up whatever may be the issue. Has anyone tried blocking off the ports on the master cylinder to see if it will hold a pedal? Where would you get the connectors to block off the ports??
Laughing Gas
Joined: Jan 2006
Posts: 1,098
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From: Minnesota
If you have no leaks anywhere, and the pedal still sinks, it has to be the master cylinder leaking internally...where else could it leak? You can block the ports by buying some short lines of brake line with the fittings on the end. Cut the lines off so they are shorter and just bend them over tight. I had this same deal a few years back, but always a low pedal. What I did was clamp off the rubber line with a vice grip with rubber over it...on each wheel then try the pedal. IIRC it was a front caliper that was the culprit even though it bled clear. Good Luck...it can be frustrating.
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Fleet Mechanic
Joined: Nov 2000
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From: In my House
I am still using the old abs module...there is nothing wrong with it. It is in the valve body (supposedly). An update though, I decided to make a plug and plug off the rear port on the master cylinder, and I did get a solid pedal, no sinking, so I figure the problem MUST be in one of the rear calipers....The only one that I had touched originally was the pass side, so I replaced and bled the pass side.....still the same