97 GMC 1500 can’t get brakes bled

[El Campo]

Trying to bleed brakes after changing brake hose and cannot get the large bubbles to stop or a meaningful amount of fluid to come through hose in a reasonable time. Tried to do with mityvac which I understand might not be best option. Depressed the button on abs module and has good oil flow from it when pedal is depressed. Do not have the abs scan tool. Tried driving it and locking up wheels, didn’t help. What do I do next?
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[Mr_Rich]

There's a $20 tool you need that clips on the valve at the bottom of this pic I posted. You can then bleed the brakes like normal. It's what I had to do when I replaced the MC and reflared and refitted some of the brake lines on this K2500 I'm working on.

[speedygonzales]

Changed a few master cylinders on my truck AND brake lines and never needed any special tools.

I had tried a mity vac in the past and it was terrible because it drew air from around the brake bleeder screw. The only method I use now is put a tight fitting hose on the bleeder screw and immerse it in a clear bottle of brake fluid. You can then open the bleeder and pump the brake pedal till you get either clean fluid or no more bubbles then close the bleeder. As long as the hose stays in the fluid, you will not get air in the system.

Save the mity-vac for other jobs, this isn't one of them.

Oh and if you want better brakes, change your MC if it looks like the one in the photo above.

[83GMCK2500]

The '97 will require a bi-directional scan tool to properly cycle the ABS unit and fully bleed the brakes.

[Tom]

I've replaced the master cylinder with a stock and now a NBS version, all 4 brake calipers [disk rears] along with wheel cylinders back when the rear was drum, and all the hoses with steel braided lines. I've never needed a special tool. I've never cycled the ABS pump either. Master cylinders do go bad on occasion remember.

[speedygonzales]

Originally Posted by Tom

I've replaced the master cylinder with a stock and now a NBS version, all 4 brake calipers [disk rears] along with wheel cylinders back when the rear was drum, and all the hoses with steel braided lines. I've never needed a special tool. I've never cycled the ABS pump either. Master cylinders do go bad on occasion remember.

Same here and I have a Solus Pro and never needed to use it.

[83GMCK2500]

Originally Posted by Tom

I've replaced the master cylinder with a stock and now a NBS version, all 4 brake calipers [disk rears] along with wheel cylinders back when the rear was drum, and all the hoses with steel braided lines. I've never needed a special tool. I've never cycled the ABS pump either. Master cylinders do go bad on occasion remember.

Originally Posted by speedygonzales

Same here and I have a Solus Pro and never needed to use it.

It is possible that when you've done your brake repairs/upgrades that you did not have any air get trapped in the abs modulator. OP didn't say how long he had the system open for, but if he did get air trapped in there it could be why he is experiencing the reduced flow and continued bubbling. Yes, a bad MC can also do that, not discounting your suggestion. I know on my '96 when I upgraded the front to GMT800 brakes it stopped better but it still wasn't "amazing" like other reviews were. I made an appointment at dealer, had them bleed the brakes, and then about choked myself with my seatbelt when I tested their bleed job. Same with a '97 K1500, all new cal/pad/rot, cyl/shoe/drm and they were better but not impressive...dealer bleed and nose print on the steering wheel.

See attached snip from Alldata.

[speedygonzales]

Originally Posted by 83GMCK2500

It is possible that when you've done your brake repairs/upgrades that you did not have any air get trapped in the abs modulator.

OK I'm with you there but if there was not air trapped as you suggest, why would ours still be crappy as it was before?

I'm still going to look into cycling the ABS and bleeding it completely and see what happens.

Stay tuned for an update when weather in SE VA permits.

[speedygonzales]

OK so after having the truck since I bought it new in Dec 1998, With the persuasion of this post I did it. For the first time I did the "automatic brake bleed procedure" as listed on the menu of the Solus Pro.

Holy crap the brakes are back as good as new. I'll be posting elsewhere to let folks know.

thanks to 83GMCK2500 and a guy on a GTO bulletin board to spark my curiosity.

[83GMCK2500]

Hallelujah! I'm very happy for you, congratulations in restoring your braking!

Regarding your question in previous post, I couldn't say...I recognize that a person would need a fairly detailed education in hydraulic systems for full analysis. The classes I've had only scratch the surface, enough for me to know that there is a lot left to learn. Without seeing a diagram that showed the various states each portion of the system is in when the brakes aren't applied, are applied, and with/without ABS cycling it'd only be a guess. My guess: when the pedal is applied and the abs system isn't directing a lock/unlock event the trapped air is compressing or allowing fluid to bypass the valves in the abs valve.

[Mr_Rich]

I've never had a problem bleeding the brakes on my '89, but the '97 with 4WAL brakes is another story. I bought the J tool to assist in the brake bleeding task.

I came across this prose about two years ago. Editorial at the end is not mine.

KELSEY-HAYES 4WAL BLEEDING PROCEDURE

Brakes can be bled in the usual way manually or with pressure or vacuum equipment. The wheel bleeding sequence is RR, LR, RF, LF. If a pressure bleeder is used, the combination valve must be held open.

If the EHCU Modulator or accumulator have been replaced, or air has entered the ABS part of the system, the EHCU modulator will have to be bled using one of two special procedures:

The preferred method is to cycle the ABS system using a factory scan tool or an aftermarket pro-level scan tool that has bi-directional capabilities so it can access the ABS system and cycle the pump and solenoids. A basic DIY scan tool that only reads codes and sensor data cannot do this.

If you can't get your hands on the proper scan tool, you can use the following manual bleeding procedure:

1. First, bleed the entire brake system using a pressure bleeder or vacuum bleeder. Pumping the brake pedal also works but must be done carefully so air doesn't siphon back into the lines. Bleed the wheels in this order: Right Rear, Left Rear, Right Front, Left Front.

2. Once all the air is out of the master cylinder and brake lines (no bubbles in the clear plastic bleeder hose you have attached to each of the calipers or wheel cylinders), you can proceed to bleeding the ABS system.

3. Open the internal bleed screws on the modulator one quarter to a half-turn. These are the cap screws on either side of the modulator. Note: Some newer 4WAL systems do NOT have these bleed screws so you must use a scan tool to bleed the system or try loosening the brake lines to the modulator to vent air.

4. Connect special tools (such as Kent-Moore No. J39177) to hold open the high-pressure accumulator bleed valves.

5. Open the two bleeder screws at the BPMV and bleed the unit by forcing fluid through it with a pressure bleeder or by slowly pumping the brake pedal. Do NOT allow the master cylinder reservoir to run low (add brake fluid as needed during this procedure).

6. Bleed the wheels again as before: RR, LR, RF, LF

7. Tighten the modulator bleed screws and remove the tools from the high-pressure accumulators.

8. Turn the ignition on and start the engine. Press down firmly on the brake pedal several times. If the pedal does not feel firm, turn the engine off, pump the pedal to relieve residual pressure in the system, and bleed each of the four brakes as before to get rid of any air in the system.

It's a pain to go through all of these steps, but if you do not and air is trapped in the ABS unit (or pump on applications that have a pump), it will eventually get into the lines and give you a soft pedal.









In other words, if you're doing this in your driveway and it's cold outside, welcome to hell.

[speedygonzales]

I never replaced anything except the master cylinder. It worked for me as it did for many others that I read about that didn't change ANYTHING.

Just do the Automated Bleed Procedure and be done with the issue for good.