Soft Brakes, New Day

[nick0789]

2005 Z71 tahoe with hydroboost

Issue
soft brakes for the first 2-3” of pedal travel then I finally start getting bite.

When did this start?
It started earlier this year after I replaced the hydroboost unit (new GM) and master cylinder (new GM). Why? I flushed my power steering pump and ended up springing a leak in the inner seal of the hydroboost unit and started with the softer brake feel.

Did you bleed the brakes after this?
sure did. I went through the whole process twice and didn’t observe any additional air coming out at any of the calipers on the second pass. First pass was done with Motive pressure bleeder, second pass was done manually.


I had mostly forgotten about it as my Tahoe has never had the braking potential of a Porsche, but after driving a rental all weekend on vacation and coming back to the soft brakes I knew I had to revisit it. Today I went back to the brakes and cycled the ABS unit twice (auto bleed function on my tool) and then re-bled all four corners … with no air visible upon bleeding.

The brakes feel marginally better but are still soft. What are my options at this point?
When I push the pedal and then release it springs back no issue so I was under the assumption that I had air in the lines still (or ABS unit). During normal driving of the vehicle I can occasionally get firm brakes but rarely do I get them in the mornings which makes me think (part of) my issue could be coming from whatever front brakes the PO had installed. I have had this vehicle for almost 5 years (~50k miles) and the front pads haven’t been changed during my ownership: quite frankly they still look to be 60-70% so I have no idea what friction material they are. I thought about replacing them as PM anyways but wanted to see about anything I may be missing.

Thanks!

 

[B-train]

OK, not to question your skills because they sound on par, but when you did the manual bleed did you have someone pump the pedal while you were at each wheel to Crack the bleeder under pressure?

The reason for asking is because I've been there, done that. Sometimes a second person is necessary to get a good job, instead of a machine. Again, just my experience.

My other thoughts/questions are 1) how does the truck stop? Good w/o having to stand on the pedal, or mediocre? 2)Was there any adjustment that could be made for the brake pedal rod?

If it stops better after using the brakes, it could be a friction issue, but you would think that would feel the same no matter what.

Or, lastly is the new master cylinder bypassing internally and after some use the cups get reset and work better. Try doing a QUICK brake stab and see how the pedal feels. That will tell you if the mater is bypassing or working properly. If it's bypassing and you stab the brakes, most times you'll end up with a solid pedal. Then it will go limp again when used as normal after a rest period

 

[Fless]

After changing the master cylinder it likely needs an ABS bleed procedure.

 

[nick0789]

OK, not to question your skills because they sound on par, but when you did the manual bleed did you have someone pump the pedal while you were at each wheel to Crack the bleeder under pressure?

The reason for asking is because I've been there, done that. Sometimes a second person is necessary to get a good job, instead of a machine. Again, just my experience.

My other thoughts/questions are 1) how does the truck stop? Good w/o having to stand on the pedal, or mediocre? 2)Was there any adjustment that could be made for the brake pedal rod?

If it stops better after using the brakes, it could be a friction issue, but you would think that would feel the same no matter what.

Or, lastly is the new master cylinder bypassing internally and after some use the cups get reset and work better. Try doing a QUICK brake stab and see how the pedal feels. That will tell you if the mater is bypassing or working properly. If it's bypassing and you stab the brakes, most times you'll end up with a solid pedal. Then it will go limp again when used as normal after a rest period

You’re not going to offend me lol.

I had a second helper for both the pressure bleeding and manual bleeding process, and the bleeder screws were not opened unless the motive canister was pressurized or the helper was holding the brake pedal (and it was closed before either of these two were stopped). The first time I bled brakes on my old Jeep I learned real quick about air flow lol

Stopping feels mediocre, around a 6/10. I’m not sure I’d trust it in a sheer panic stop at this point.

For the hydroboost unit I did not see anything that would have been something I could adjust with the rod. I do have the brake pedals that can be height-adjusted but I have gone through that portion in the past as well just to see.

Braking sometimes /occasionally feels better after city driving but I can’t pinpoint that it’s due to the pads being warm.

I’ll look at the master cylinder possibility in the morning.

Thanks again!

 

[nick0789]

After changing the master cylinder it likely needs an ABS bleed procedure.

When I replaced the master cylinder (however many months ago) I found a thread on the forum that showed how to bypass needing a scan tool to bleed the ABS module. It involved cycling the ignition and I did notice small amounts of air coming out during bleeding after doing this step.

I used a scan tool today to auto-bleed the ABS module. For my tool I have to hold the brake pedal down while it automatically cycles the ABS unit. I will admit that the first time I did it I thought it finally worked because the pedal pushed back on my foot pretty firmly during the first couple cycles of the auto-bleed procedure … then it went back to somewhat soft so I figured any air got pushed into the lines. Re-bleeding manually (with a helper) didn’t produce any air bubbles though

 

[Scottydoggs]

you need to lift the front wheels off the ground. engine on pump the brakes 30 times. turn it off. turn the wheels lock to lock 15 times. open the p/s pump cap before hand. lay some card board under it cause the trapped air will burp and push fluid out the top. if you leave the cap on it will still burp out past the cap, but will spray all over.

 

[B-train]

That is brilliant! That idea totally escaped me. It's the same as replacing a rack and pinion......20 years ago here, so a little rusty. Makes a lot of sense.

 

[Scottydoggs]

That is brilliant! That idea totally escaped me. It's the same as replacing a rack and pinion......20 years ago here, so a little rusty. Makes a lot of sense.

same idea, but this bleeds the booster. we got a work truck with a slow leak on ps hose. pedal gets mushy here and there. i do that and the brakes are back.

 

[Larryjb]

I remember reading somewhere about how difficult it can be to bleed air out from master cylinder. I believe that an air pocket can still reside even after bench bleeding. If I recall correctly, if you park on a hill facing down so that the rear of the master cylinder bore is pointed slightly up, and then bleed it, you can get out some of that trapped air.

Found an article about it here: https://www.brakebleeder.com/removing-trapped-air-angled-master-cylinder/

 

[BoatyMcBoatface]

I know this may sound odd but i had a problem with another vehicle with the same crappy feed back of the brake pedal. I tried a few things and once i changed out the pads to semi metallic. used a name brand fleet rated and the brake response was immediate. I'm not saying that's your issue but organic cheapo pads even with high pressure behind them leave a soft braking feel. I would also make sure your hydro boost is working correctly. With the vehicle off pump the brakes till the pedal gets firm. Keep your foot on the pedal and start the vehicle, the hydraulic pressure should start to push the pedal back up as it builds. We have a Lincoln and i just replaced the pads and the previous pads gave me the same lackluster feel. I the pedal would go quite a bit and even standing on the brakes i was sure they would never lock up. I put a set of semi metallics all the way around and a toe on the pedal makes it stop on a dime.