Bad Master Cylinder?

Discussion in 'General Automotive Questions' started by Jimbob, May 8, 2017.

  1. Jimbob

    Jimbob SITE SUPPORTER- SILVER GOLD MEMBER

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    So I'm sitting at a light about a mile from my house after a 4 hour trip home in my 1997 Chevy C1500. Just as I'm about to let off the brake, I feel the pedal sink. Thinking i have lost brakes, I pull into a slide road. I can push the pedal to the floor but can still stop the truck, but something definitely was wrong. Half way back home the brake warning light comes on. So today, checked all over for leaks, nothing. Even checked the back of the master cylinder, but still no leaks to see. Pumped the pedal a few times, but the brake warning light doesnt come back on. I can hear/feel rear brakes tightening up but not the fronts. I noticed then, that when I push the pedal down, brake fluid comes up into the larger reservior, so am guessing a seal has gone in the MC and the fluid is going back into the reservior and not to the wheel cylinders. Anyone suggest any other tests I can do?
     
  2. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    For how inexpensive one is, a new M/C and fluid flush is a good idea anyway. I would start there and work my way out.
     
  3. 1965 Ranchero 66G

    1965 Ranchero 66G In Maximum Overdrive PLATINUM MEMBER

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    Power brakes ? If so look under the dash where the brake rod goes through. Sometimes the rear seal on the master goes out & dumps through the power booster & see if it is wet. Just a thought.
     
  4. 1965 Ranchero 66G

    1965 Ranchero 66G In Maximum Overdrive PLATINUM MEMBER

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    Have to agree with Andy, M/C' S are cheap.
     
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  5. Basstrix

    Basstrix In Overdrive BRONZE MEMBER

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    Sounds like the forward seal in the MC is leaking. Sometimes you catch them when they first start leaking and the light application of pedal pressure is when the pedal will drop rapidly....they are self-energizing seals, so when you apply pressure rapidly (at least at this stage), it'll probably seal better...but the time will soon come when it won't matter.
     
  6. ribald1

    ribald1 In Maximum Overdrive PLATINUM MEMBER

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    Fluid pushing up in the cylinder reservoir is usually a sign of air in the system.
    A failing caliper seal can draw air in as it recedes without leaking fluid out.
     
  7. Jimbob

    Jimbob SITE SUPPORTER- SILVER GOLD MEMBER

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    I hear ya on the MC swap out anyway, i guess i will have to do that at some point anyhow, was kinda avoiding it on this truck though as it has one of those ABS systems that you need the Tech 2 scanner to bleed properly. I have read that you can just find a gravel road however to activate the ABS on and then rebleed.
     
  8. burninbush

    burninbush In Maximum Overdrive

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    Probably a master cylinder -- but if that doesn't fix it, take a closer look at the equalizer gizmo. Those can plug up and stop fluid flow to either end of the car.
     
  9. Mike1969

    Mike1969 In Third Gear

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    Funny, my Dad's '68 Galaxie wagon and '69 F-250 both have the same issue you just described. I happened to be driving the F-250 when I felt the pedal sink and it scared the daylights out of me!
     
  10. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    Is the ABS RWAL or 4WAL? RWAL has the controller module in a bracket at the master cylinder; it does not need a Tech scan tool to bleed; only the 4WAL does. The RWAL unit has a bleed screw or two on the body of the unit, which is on the driver's framerail, below the cab floor. Which do you have?
     
  11. ribald1

    ribald1 In Maximum Overdrive PLATINUM MEMBER

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    Good old ABS braking.
    What everyone wants, more things that can fail in the braking system.
     
  12. Jimbob

    Jimbob SITE SUPPORTER- SILVER GOLD MEMBER

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    I have the 4WAL Andy...

    I also read that another way to bleed the ABS, is to raise the rear of the truck, stick it in Drive, get the rear wheels up to about 60mph and slam the brakes on, in theory tricking the ABS to come on.
     
  13. Jimbob

    Jimbob SITE SUPPORTER- SILVER GOLD MEMBER

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    Hadnt checked that, i did unbolt the MC from the booster however, but couldnt see any seepage out the back. Im convinced its probably a MC internal seal as Basstrix suggests, i dont see any reason why fluid should enter the reservior on the pedal press, it should all the going to the wheels in my mind. Have a new MC on order anyhow from RockAuto so ill see how it goes, just hope i can get the system bled.
     
  14. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    Sorry, doesn't work. All four wheels have to be moving before the ABS will actuate. Running down a gravel or dirt road, while dangerous, would actuate the ABS.
     
  15. plumcolr

    plumcolr In Maximum Overdrive

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    You don't have to drive too fast or dangerously. Find a quiet road with a dirt shoulder put two wheels (left or right side) and step firmly on the brakes, just hard enough to make the abs kick. 20mph should do it.
    Just went thru this on a 97 suburban K1500. That one has a Kelsey-Hayes 3 sensor system (not a 4wal) It was a bitch to troubleshoot. Bad combination valve.
     
  16. Jimbob

    Jimbob SITE SUPPORTER- SILVER GOLD MEMBER

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    Sounds like the exact same system i have. So did you have the same symptoms i had? How do i trouble shoot the combo valve? I have the new master cylinder now, so am going to try and get it installed this weekend if i get time. So when you bleed your brakes, my understaing is is they will bleed and you can get a decent pedal until the ABS activates and then any air in the ABS gets purged into the lines and rebleeding gets rid of that. Do you also find that your pedal is really low on these trucks? my pedal would travel 1/2 way down before anything happened but always had good brakes before even with the low pedal.
     
  17. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    When you do install the MC, bench bleed it first, then pressure bleed it at the MC connections. Any air below the MC connections but not yet in the ABS unit should work their way out as you do the bleeding.
     
  18. plumcolr

    plumcolr In Maximum Overdrive

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    Yeah. The suburban is my wife's. Pedal was always very low, disconcerting first time you drive it. Found an article that said GM released a TSB for suburbans because they are heavier in the back than a pickup. TSB said basically if you can't stand the customer bitching any longer replace the MC and combo valve with (different part numbers). I did that around 2004 or so, raised the pedal a good oh I dont know a quarter inch?? when brakes applied.

    It's also possible the ABS unit itself is bad and sucking in air. If you get decent brakes and disable the ABS, do the brakes go soft on you again?

    And I had my first experience with a syringe type MC bleeder. I like it. Much easier than trying to pump the MC during a bench bleed.
     
  19. plumcolr

    plumcolr In Maximum Overdrive

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    Let me expand: If you disable the ABS and they go soft on you again it's probably as Ribald said a bad caliper or wheel cylinder seal (fine with pressure, pulls air when pedal released), or a bad combination valve. This AFTER you have replaced the MC which it sounds like you are going to do.

    If you disable the ABS and pedal stays good, the ABS is pulling air when surging brakes. Good luck with that. I don't think you can find a new one. Or a reman. You'd probably need to eliminate it (I refuse to use junkyard brake parts even if it is my wife's truck :rolleyes:).

    If that is OK, then if after several bleedings the same thing happens, disable the ABS, get good pedal and live with the dash lite. Dunno about you but I have more miles by far non-abs than abs.
     
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  20. 1965 Ranchero 66G

    1965 Ranchero 66G In Maximum Overdrive PLATINUM MEMBER

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    If it works, & your comfortable with no abs, pull the bulb out of the dash.
     

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