1999 Tahoe, soft brake pedal

[Miami-Dade]

@99brokeTahoe Moved you here.
Welcome from Queens NY.

 

[99brokeTahoe]

Best fix I found on OBS trucks is to use a NBS MC, makes the pedal feel like you're actually going to stop. I must just be lucky but I've never had any problem bleeding these brake systems and I do have a scan tool that can do the ABS activation but never see any difference if I do it that way or without.
Test on a booster is to pump the pedal multiple times with the truck off. Then while holding pressure on the pedal start the truck and you will feel if the booster is working as the pedal will drop.
The simple test on a MC is buy 2 plugs for the MC, thread them in and step on the pedal. This will let you know if you have air in the MC or just how high the pedal can be if the system is good. Of course every wheel's brakes should be inspected and all of the lines especially on a 25yo truck.

I've never had a problem bleeding brakes like this before, unless there was failure in the system, and I had hard time getting the portioning valve centered. Everything is new within the last year or two as far as the brakes go. I did try my Tech2 to do the automated bled procedure. It wouldn't because it had speed sensor codes. I cleared the codes, then it worked. I did some research on the codes, and found a TSB on the speed sensors, that sounded like the problem I was originally trying to fix. After reading this TSB, I think it was the ABS trying to activate, but not like it does at speed in loose gravel. About a month ago, I replaced both CV axles and wheel bearings. Both axles, bearings, and bearing hub bolts were difficult to remove. Rust and age I guess. I even replaced the hub bearing bolts with new, and coated everything with antiseize. The TSB talked about dirty wheel speed sensors, and rust on the mounting surface causing the low speed ABS problem. Last night I pulled the front wheels, rotors, and calipers to check the sensors. Both sensors, and the reluctor wheel teeth had antisieze on them. I think that this is what caused my original problem. I cleaned everything, put it back together, did the automated bled again. The pedal doesn't feel any different, and I plan on bleeding all 4 corners today again. What kind of plugs would you use in the master cylinder? Maybe I'm overcomplicating it, but I thought that they were an odd straight metric thread with an odd bubble flare, is the reason I ask. Thanks for all the replies.

 

[Eman85]

There is no such thing as an odd strange thread, it's all measurable and once you know the size you just buy a plug that size, I don't even think they are metric. I can't remember the size but if you look up MC specs it should give them to you.
I've had problems with ABS activation after hub replacement, mostly from replacement hubs with too much play or runout, ABS will see different speeds between the 2 front wheels usually just before stopped and it's hard to see on a scan tool. Cleaning the surface where the ABS sensor mounts is important. Also be sure to use a torque wrench tightening the axle nuts, not an impact.

How to: NBS master cylinder swap for firm brake pedal

How to swap out the squishy brake pedal for a firm brake pedal. The OBS trucks are known for a horrible squishy brake pedal. -NBS 1500 series master cylinder (buy the corresponding master cylinder for your truck, 1/2 ton=1/2ton, 3/4ton= 3/4ton) -AGS BLF 26C brake line adapter (I got mine at...

www.gmt400.com

Here's a link to everything you wanted to know about fitting size and MC's.

 

[99brokeTahoe]

I understand. They just don't look like regular hardware store flare fittings, and I didn't know what I could use that wouldn't damage the sealing surface.

 

[Eman85]

https://www.amazon.com/Master-Cylinder-Plugs-Compatible-Inverted/dp/B0CCXMT9JZ/ref=pd_ci_mcx_pspc_dp_d_2_t_1?pd_rd_w=y2Ijy&content-id=amzn1.sym.568f3b6b-5aad-4bfd-98ee-d827f03151e4&pf_rd_p=568f3b6b-5aad-4bfd-98ee-d827f03151e4&pf_rd_r=JR2RQCM696GNDBXBSTT8&pd_rd_wg=JFPTN&pd_rd_r=12265358-dd9a-47e5-acac-5e4df009c13a&pd_rd_i=B0CCXMT9JZ

A parts store should have them but there is always Amazon or ebay.

 

[99brokeTahoe]

Did you get the little rod that goes into the brake booster out of the old one and install into your new one? Is your brake light coming on? I suspect you still have some air trapped at your ABS manifold because you didnt use an auto bleed feature of a bidirectional scanner. Seen this scenario right her on this forum so many times.

Old and new push rod, side by side. Looking back, I think I caused the original problem when I replaced both front hub bearings, and contaminated the sensors and tone rings with antisieze. I thought it was something binding in the linkage, MC, or booster, but it was just ABS coming on a slow speeds. I think the problem is in the ABS module now, but I'm going to try and bled it again. Thanks for the reply.

 

[99brokeTahoe]

I did automated bled process with a Tech2 scan tool, then manually bled all 4 corners after, 3 times. No change in how the pedal feels. Just to be clear, the pedal is firm with the engine off, and doesn't "pump up". When running, it will go to the floor, but will lock the front wheels before it ever gets to the floor. Just to note, doing the automated bled process would actually push the pedal back up off the floor. I'm going to try a used ABS module/manifold today when I get home. There is no brake light on, and there no codes either. Truck has 290,*** miles, and I believe I created this problem with the contaminated speed sensors, and the ABS module is either damaged, or just wore out, and something is bypassing internally. In the past 2 years, I've replaced calipers and lines. Never had a problem bleeding it. I've replaced both wheel cylinders at a different time. Never had a problem bleeding it. Changed both hun bearings because I hadn't done that yet, and because I was replacing a CV axle. Right after that is when I thought my pedal was binding, so I replaced the master cylinder. Didn't have a problem bleeding it then either, and brakes felt better than they ever had. Last thing I did was replace the booster, again because I thought it was binding. I didn't open the hydraulic system to replace the booster, but right after that is when I started getting the low pedal. So, with all that being said, I'm going to replace the ABS module off another truck I have, that is "known good". Again, thank you all for the help.

 

[Eman85]

Why not put the old booster back on. Most parts purchased today aren't as good as what is being replaced which is why replacement instead of diagnosis usually puts people further down the well.
If you want a really improved pedal do the NBS swap.

 

[99brokeTahoe]

Old booster was turned in for core charge at the local NAPA. I think it was coincidental that my pedal went soft changing the booster. I didn't get a chance to swap the ABS module, but I tried something else. I'm not saying this is fix for every low/soft brake pedal either, but it made a big difference in how my brake pedal feels. I simply unplugged all 4 electrical connectors. I started the truck, and now have both ABS light and BRAKE light on, witch haven't been on at all. This made the biggest difference in pedal feel to me, I would say 75% of what it was before I started having any problems. No, this didn't fix anything, but it did make the brakes feel a lot better, and makes me more hopeful that changing the ABS module will fix my problem. Just a quick recap, I think I had the low speed ABS sensor problem. My brake pedal with the engine running will go to the floor. I've bled it with scan tool, sliding the tires on loose gravel, and bled again. Unplugging the ABS module made the biggest difference in my particular truck as far as brake pedal feel. Yes, both ABS and BRAKE lights came on, and yes you can lock the wheels up at will, but the brake pedal doesn't go to the floor like there's nothing there. I'm going to try and swap the module tomorrow night, and hopefully, that fixes my problem.

 

[east302]

Just curious - are you just swapping the module (black part with Kelsey Hayes on it) or the whole thing with the valve assembly and combination valve that the lines go into?

Since it is improved with the module unplugged, do you think the issue is with the controls rather than something mechanical in the valve portion?