1998 Yukon Brake problem?

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Koby

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On my '98 Yukon slt 4x4, My brakes felt spongy. It felt like I needed to bleed the brakes. However, no matter how much I pumped the brake, no fluid, bubbles, or even air, would come out of the bleeder nipple. I fugured it might have been the cylinder inside the brake drum causing the problem, so I disconnect the brake line from the drum entirely. I try pumpkin again, only to get the same result. I asked my dad what the problem might be, and he said it could be a clogged/pinched line, the abs system, or the master cylinder. How would I go about testing these theories? If I needed to get a new master cylinder, would I need a new reservoir or can I reuse the old reservoir on the new cylinder? My dad also said that we could bypass the abs system lines entirely, and just live without abs (I'm sure the system doesn't work anyway, the abs light has been on since we got the truck a several years ago). Any help is much appreciated.

(Note: I tried bleeding the rear drum brakes. I am certain that the front ones get fluid because they stop the truck when I press on them, I just get the spongy feeling from the brake pedal (sometimes the pedal gets harder to push when I repeatedly press on it, but sometimes it never build up resistance). Also note that I am not currently driving this truck on the road, so don't freak out about me driving around with only front spongy brakes)
 

drakon543

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It sounds like your master is dead. Some master cylinders come with the reservoir some dont so yes you can reuse the reservoir. As far as the abs system i would just have it diagnosed with a proper scan tool before you just yank it off. If a scan shows something cheap and or simple to fix i would just fix it.
 
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Koby

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It sounds like your master is dead. Some master cylinders come with the reservoir some dont so yes you can reuse the reservoir. As far as the abs system i would just have it diagnosed with a proper scan tool before you just yank it off. If a scan shows something cheap and or simple to fix i would just fix it.
I think that's what I may end up doing, replacing the master cylinder. As for the abs, there's no telling what's wrong with it, this truck hasn't been taken to a shop since 2013, as far as I know of, and I don't trust or know any mechanics so I may get the codes read at autozone or another parts store and see if it is something simple. I won't be reading codes anytime soon though, this truck needs a bit of work before I'm willing to drive it anywhere. Thanks for the reply!
 

drakon543

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Any shop can do a quick code read for you and some of the smaller shops will do it for free. All you need is the abs fault code to point you in the right direction. Until then check and make sure you pull the abs fuse until your ready to scan it.
 
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Koby

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Any shop can do a quick code read for you and some of the smaller shops will do it for free. All you need is the abs fault code to point you in the right direction. Until then check and make sure you pull the abs fuse until your ready to scan it.
I'm going to replace the master cylinder and see if that is the problem. The master cylinder seems to be a very common problem with these trucks. When I get it to driving condition, I will check the abs at a shop somewhere to see if theres a problem with it at all (other than a sensor, which is the reason for an abs light 99.9% of the time.
 

Orionx19

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Could possibly be the master cylinder but replacing parts to find out is the hard way and the expensive way to diagnose, you can make up some plugs to plug of the master cylinder out of brake line fittings, bleed it and see if your pedal is firm, if so then your problem is likely the accumulator which is in the abs hydro electric module, you can bypass that like your wise dad suggested and see if you then have a firm pedal.. I would not suggest to drive it without the anti lock brake system working, it really does make panic stops much safer. An hour or so of some logical diagnostics can save you some grief and $$
 

Orionx19

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Could possibly be the master cylinder but replacing parts to find out is the hard way and the expensive way to diagnose, you can make up some plugs to plug of the master cylinder out of brake line fittings, bleed it and see if your pedal is firm, if so then your problem is likely the accumulator which is in the abs hydro electric module, you can bypass that like your wise dad suggested and see if you then have a firm pedal.. I would not suggest to drive it without the anti lock brake system working, it really does make panic stops much safer. An hour or so of some logical diagnostics can save you some grief and $$
Also to note; if the ABS light is on, by default the system is disabled and cannot operate, it will have to be diagnosed properly if you want it to work
 

east302

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Is the brake warning light on?

The combination valve is attached to the ABS module and regulates pressure to the front and rear brake circuits so that the discs and drums work in tandem. If there is a large pressure differential in the front vs rear brake circuits (such as a fluid leak), the valve will close, isolating that low pressure circuit so that fluid doesn't just pour out leaving you with no brakes. The pressure switch will then signal a warning light on the dash.

IMG_3535_zpspybxjfid.jpg

The combination valve is on the lower right in the photo with two lines coming in from the master cylinder. The Brake Pressure Modulating Valve is the piece with three lines coming out (two for front, one for rear). This is what the ABS module uses to control the brakes during ABS activation. The pressure switch is the black plug between the front/rear lines.

There is a pin underneath a rubber cap on the side of the combination valve. When you press the brakes, the pin pushes out perhaps 1/8". Try holding the pin down while someone presses the brake pedal. Assuming that there is no leak in the system, this may allow the valve to reset and send fluid to the drums.

As for bypassing the abs, if you disconnected the lines from the abs unit, you'd also be bypassing the combination valve. You need that for proper brake function and would need to duplicate it. With a properly functioning system, you can get a good brake pedal with these trucks.
 
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east302

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Could possibly be the master cylinder but replacing parts to find out is the hard way and the expensive way to diagnose, you can make up some plugs to plug of the master cylinder out of brake line fittings, bleed it and see if your pedal is firm, if so then your problem is likely the accumulator which is in the abs hydro electric module, you can bypass that like your wise dad suggested and see if you then have a firm pedal.. I would not suggest to drive it without the anti lock brake system working, it really does make panic stops much safer. An hour or so of some logical diagnostics can save you some grief and $$

Good suggestion. In case you don't have spare brake fittings, I've seen plastic plugs at parts stores that should do the trick. Look for a master cylinder bench bleed kit, it should have what you need.
 

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