Low/no brake fluid to rear brakes

[Maverick6587]

I was bleeding the brakes and flushing the system to clear out the brake fluid, it was a dark green color. After bleeding the brakes the pedal is softer now than before we bled the brakes.

I found out why the reservoir cap wouldn't tighten. It was missing the gasket/seal for the reservoir cap. Cheap and easy fix.

While attempting to bleed the rear brakes by letting there fluid flow. There was almost no flow coming out of the bleeder screw. Even when I had my dad put pressure on the brake pedal, there was almost no flow at all. We had full flow coming from the front brakes. Both rear had almost no flow though.

I'm thinking that either the master cylinder is plugged or more likely the ABS module is plugged?

I have a spare master cylinder I plan to bleed and install.

Can we put air pressure into the ABS module to try to clear the lines? If I have to swap out the ABS module from the junkyard from there exact year before, does that have to be programmed (is it VIN coded or just vehicle specific)?

I'd we remove/unplug the ABS module, do we need to bleed the brakes using the Tech 2?

 

[Mooseman]

Have you tried loosening the brake hose from the caliper? Could be the bleeder is plugged. I've seen that before. Since you already have the new MC, might be worth it to just install it to eliminate that as the source.

 

[Maverick6587]

I unplugged the left rear hose from the caliper and there was almost no fluid coming out even with pedal pressure.

We're swapping out the master cylinder right now. Working on bench bleeding it right now.

Is there a reset button for the ABS module? A guy at AZ said there's a reset button, for when ABS detects a low/no fluid to the rear.

In that when it detects what it thinks is a leak, it will stop brake fluid from flowing to the front or rear so that you still have one or the other to be able to brake still. That's how he explained it at least.

 

[Mooseman]

Is there a reset button for the ABS module? A guy at AZ said there's a reset button, for when ABS detects a low/no fluid to the rear.

In that when it detects what it thinks is a leak, it will stop brake fluid from flowing to the front or rear so that you still have one or the other to be able to brake still. That's how he explained it at least.

Never heard of that one. Only thing are code clearing and ABS bleeding using a scanner.

 

[Maverick6587]

Replacing the master cylinder helped a little bit. We had to use a vacuum bleeder to get the air out of the brake lines. I have brakes again though.

I think we 2 man bled the lines about 10 times and got the brakes back on 1 bleed with the vacuum pump.

 

[m.mcmillen]

Is there a reset button for the ABS module? A guy at AZ said there's a reset button, for when ABS detects a low/no fluid to the rear.

He was probably thinking about a proportioning valve. Some of those had a button to push to reset it. The proportioning valve is built into the ABS unit now.

 

[Maverick6587]

I went through 2 1/2 quarts of brake fluid for the bleeding. I still have what sounds like a boiling liquid.

I guess there's a slim possibility that there is still air in the brakes lines. I feel the boiling right where feet are on the floor. It is most noticeable as I'm coming to a stop but I have felt it after driving for 20 minutes down the highway. It "boils" for about 2-3 at a time.

It starts when I'm almost at complete stop and then when I start to drive forward. I haven't tested it in reverse. Usually twice per stop.

Could this be coming from the fuel lines or the transfer case? I did not have time to check the fluid levels of the transfer case or the differentials.

 

[m.mcmillen]

Sounds like you have a bad speed sensor. The abs module is seeing a wheel speed that is not the same as another and engaging the ABS. You mentioned a Tech 2 earlier - If you have one, or a high end scan tool, watch your front wheel speeds as you come to a stop and take off. One of the front sensors is likely your problem, at least for the noise.

 

[Maverick6587]

I do have a tech 2. I am getting a traction control error randomly. I have a tpms error as well. Tomorrow I was going to take a break from all the work into the new ride.

I guess she requires a little more work though! I did my initial bluedriver scans today. There were a lot of codes! I cleaned the two left side frame grounds hoping that would fix some of them.

I didn't have time to scan again after cleaning those grounds. I'll check them tomorrow. Great analysis @m.mcmillen

 

[Mooseman]

Did you use the Tech 2 to do the ABS brake bleed? Might help firm it up more.

Is it just a traction control/Stabilitrak error? I get an occasional Stab error caused by the SWPS. If you only get a Stab/traction control error, it's likely a SWPS problem. If you get it with an ABS light, that's more likely a problem with a wheel sensor or the ABS module because that would affect both systems. The SWPS only affects Stab/trac.

 

[Maverick6587]

I left the Tech 2 at home this weekend. I'll bring it with me next weekend to do another brake bleed and install new pads.

It is a stabilitrak error I get. I don't remember any ABS errors, but there's been a lot of figuring out, what's what, while driving. I could have missed it or not noted it though.

I think I have about 10-20 DTCs I have to figure out so far. I'm hoping those grounds were the cause of most of them. They were pretty corroded/rusted.

 

[mrrsm]

From 12:02 onward... (Courtesy a prior Thread entry via @Mooseman) Dr. Shock demonstrates the use of his Tech 2 for an Excellent Demo of ABS Brake Bleed Procedures from his YT Channel:

As for the weird Green Color of the Old Brake Fluid. The "Thirsty for H2O" nature of this Organic Stuff makes it susceptible to absorbing atmospheric water and contaminating the contents of the Master Cylinder. Like anything else organic... Once opened Brake Fluid has a very limited shelf life and this is true for Brand New, Un-Opened Brake Fluid.

When absorbed water separates inside the Brake Lines... anything Coppery will dissolve into the brake fluid stream as well as gradually rusting out the Mild Steel lines from the inside out. Checking the "Use By" Date on the Auto Parts Store Shelves prior to purchase is helpful to avoid buying Old Stock of the DOT-3 Brake Fluid.

 

[Maverick6587]

I used the Tech 2 to auto bleed my dad's Silverado, which was fairly simple. On the other hand, I was left in dismay at trying to auto bleed my 97x using both the Envoy and Rainier selections from the Tech 2.

I was given a menu of items to select with almost no options after selecting the brake component items. I could turn them on and off but, I was never shown an option to actually have the Tech to automatically bleed the ABS system. I never did find a way to auto bleed the ABS. Maybe another day.

Anyway, after about an hour of playing with the Tech 2... I decided to attach the rear wheel speed sensors per @m.mcmillen suggestion. I know the previous owner recently replaced the right-front hub and I replaced the left-front hub the weekend prior. So, I removed and cleaned both rear wheel speed sensors. They had A LOT of gunk surrounding them.

My plan was to actually drain the axle fluid, replace the bearings, brakes, and seals to the rear axle but, I had all of those items shipped to my home address and not my parent's house where I was working. So, next weekend I will be taking care of all of those items.

After I cleaned the wheel speed sensors though... I now have full brake power/stoppage and the ABS stopped turning on. no feeling at all like the brakes are spongy. That lasted for about 5 minutes, which has not happened before though. I think because the fluid in the axle fluid is so old and dirty, the magnets probably gathered up gunk very quickly.

I am at a loss as to why I have a full firm pedal now though from cleaning those speed sensors. It is a very dramatic difference in pedal firmness!

Edit: @MRRSM I did follow the video you posted for Tech 2 auto bleeding and that is exactly how it is done for at least Silverados. Thank you for that! My dad's Silverado is the same boat as my 9-7x was. He is going to attempt the wheel speed sensor cleaning today. I'll do some research and find something for trailblazers/envoys/etc. I do remember seeing one where he bled the system using a laptop with a Tech 2 and his wife assisted him, I just cannot find that video now though.

 

[mrrsm]

The reason that the Brake Pedal can so suddenly go from Hard to Soft is due to the Incorrect Activation of either the (L) or (R) ABS Sensors mistakenly Triggering at low speeds. When this occurs during NON-CONDITIONS just as though it was happening under the REAL CONDITIONS, the "Mis-Firing" ABS Unit prevents the Driver from Mashing Down TOO HARD on the Brake Pedal and Risk Locking Up The Brakes. On Ice Covered Roads... This condition could send the vehicle into an uncontrolled skid. It is ABSOLUTELY IMPOSSIBLE to Steer ANY Vehicle on Slick Surfaces ...if the Wheels Stop Rotating.

So in your situation, once you cleaned up and re-established the Alternating Current Voltage Signal back to around 450 Milli-Volts ... the Brake Pedal Response returned to becoming Normal and Rock Hard. This Video give a visual demonstration that echoes what you just did to your Dad's Silverado. ALL of these problems can happen as a result of the ABS Malfunction... and NOT involve the actual Hydraulic Capabilities of the Physical Braking System at all ... IF....it's in Good Repair, having a FULL Master Cylinder, Air Bubbles Bled from ALL Brake Lines and Calipers and with having Decent Discs and 'Meaty" enough Brake Pads:

As for the ABS Brake Bleeding Procedures with the Tech 2... Try removing the Electrical Connector to the ABS Unit and then Spraying out the whole setup of Male and Female Connector Points with some CRC QD Electrical Solvent, Then plug those items back together. Check the Battery Voltage with a DVOM and look closely at the Voltage Draw while the Key is in the Ignition and Turned to the On-Run Position (NOT Start) while studying the Meter for any Low DC Voltage measurements at the Battery Terminals.

The Tech 2, the PCM-BCM and ALL Vehicle Modules are very sensitive to insufficient voltage events. So set up the Tech 2 just as before and while going through ALL of the relevant menu selections... if the first ones you chose during the ABS Brake Bleed Activation Test do not work... Then Start all over again and try ANY Alternative Choices on screen to see if any difference happens that will cause the ABS to Run and Vibrate the Brake Pedal with your foot pressing down HARD during the Test.