Issues After Hydroboost Swap

[Jesse_Wenting]

So around 2 years ago I realized my parking brake wasn't holding my 98 K1500 in place when parked on a mild grade. She's an auto and I never use it but brakes is brakes and I don't skimp on safety. Ended up becoming a front to back brake job. Literally the only thing that didn't get replaced at the rear was the backing plates. Up front I replaced both callipers with ACDelco remaned units. All the hard lines and flexible hoses were done, swapped my a hydroboost unit from a 99 K2500 6.5td

Obviously the brakes and power steering lines were extensively bled after all this. I was able to get a good pedal feel and for around a year it was great. I started having an issue where if I were to sit on the brakes for a few seconds (like at a red light) the vehicle would stop, but the pedal would then slowly sink to the floor until the ABS bypass valve opened. I bled the brakes again and got a good pedal feel back, but I would still have this issues if applying the brakes for an extended period of time.

About 4 months ago I replaced my master cylinder reservoir cap seal (the old one looked was really wrinkled, like the fluid had gotten stupid hot) and when I did the pedal felt as good as it did after I had originally bled the system. Since then I've gotten to the point where my pedal will be at the floor with the ABS bypass valve open any time I need to come to a full stop. I took my ol' girl to my local dealership and had them specifically bleed my ABS with a TechII, pedal was decent again right after but less than 30 minutes later is was back to how it was before the ABS bleed. Apparently they got "a few good burps" out of the ABS.


I'm honestly at a loss at this point. I've started playing with the possibility that the callipers I got are low drag style, but if that were true how could I have achieved such a good pedal feel initially? I have noticed a little fluid on the master cylinder since I went to the dealership, appears to be coming from the reservoir seals. No leaks at any fittings or hoses, and all my bleeder valves are tight. Any ideas are greatly appreciated, sorry for the long read.

 

[Supercharged111]

If you don't have any externally leaking components, my money is on the master having an internal leak. I've had one fail like you describe before.

 

[df2x4]

X2. Really sounds like a failing master cylinder to me.

 

[Jesse_Wenting]

X2. Really sounds like a failing master cylinder to me.

Something I have noticed is after the truck has been sitting a few hours on an incline, the brakes feel really good the first time I use them. Don't suppose it might just need to be bled itself?

On bit of a side note; how do I dial in the proportioning valve on my ABS? I'm referring to the one at the bottom rear of the unit on the driver side that's covered with the little rubber cap. I do not have a valve on the passenger side of the unit.

 

[Supercharged111]

Something I have noticed is after the truck has been sitting a few hours on an incline, the brakes feel really good the first time I use them. Don't suppose it might just need to be bled itself?

This line of thinking implies that, under the right conditions, the system is able to bleed and un-bleed itself. I don't have an explanation for that, but if with constant input the pedal sinks to the floor, and you don't have fluid pissing out your wheel cylinders, then IMO it really sounds like the master.

 

[df2x4]

Something I have noticed is after the truck has been sitting a few hours on an incline, the brakes feel really good the first time I use them. Don't suppose it might just need to be bled itself?

I'm no expert so take this with a grain of salt, but I doubt it. Sinking pedal implies to me that something isn't sealing properly.

I had a master cylinder fail in my Monte Carlo this same way. Great pedal and stopping power initially, then the pedal would sink to the floor slowly to the point where the car would actually start rolling again eventually. New master and the problem disappeared entirely.

 

[Jesse_Wenting]

I had a master cylinder fail in my Monte Carlo this same way. Great pedal and stopping power initially, then the pedal would sink to the floor slowly to the point where the car would actually start rolling again eventually. New master and the problem disappeared entirely.

I'll experience this if I don't keep applying more pressure to the pedal. As soon as the bypass valve opens I can apply easily stop the truck, but at that point it feels like I'm doing more of the work than the booster if that makes sense

 

[Supercharged111]

I'll experience this if I don't keep applying more pressure to the pedal. As soon as the bypass valve opens I can apply easily stop the truck, but at that point it feels like I'm doing more of the work than the booster if that makes sense

With my vacuum brakes, once the pedal hit a certain point it turned into a rock and the braking force stopped increasing. Did you swap in a HB pedal too? My theory is that the vacuum pedal over centers and the pin starts heading more up than in toward the master.

 

[Jesse_Wenting]

Did you swap in a HB pedal too?

Yes I did swap my pedals at the time of the hydroboost swap. Forgot to mention that with the original post lol

 

[Jesse_Wenting]

@df2x4 @Supercharged111 how hard is it to rebuild/reseal a MC, and are seal kits even a thing? I know a new one isn't that expensive, but money is a bit tight rn so I'm trying to save where I can.

The part that's really confusing me is why once the brake light on the dash lights up (I'm basing my assumption that the ABS bypass valve is opening on that light and a physical "click" in the pedal as it comes on) I seem to have brake pressure.

I tried a few things and I'm very confident that I'm not having an issue with locking up the front wheels.