Pulsating Brake Pedal While In Park

[Fitz98]

I looked at a used '04 2500 Suburban today.
Lots of little things wrong with it, but the one thing I couldn't diagnose was when applying the brake pedal in park, the pedal would shudder when first applying pressure to the pedal. Sort of felt like the anti-lock brakes would feel when engaging, but since it was not in motion, I couldn't figure why that would be an issue. No engine codes or dash lights.
The seller said they replace calipers, brake lines and rotors.
Any ideas? Something simple or something major?

 

[ripcord]

Probably the ABS control module, but that's just a guess.

 

[eljefino]

Regular vacuum brakes or hydroboost?

 

[ls1mike]

Yep it is Hydroboost I have seen a power steering pump cause that.

 

[ls1mike]

Well what I meant to say is IF it is Hydroboost, but it should be.

 

[Fitz98]

RPO Code JH6 : BRAKE, HYD POWER, 4-WHEEL DISC, 9900 LBS
Is that hydroboost?

 

[Astro14]

RPO Code JH6 : BRAKE, HYD POWER, 4-WHEEL DISC, 9900 LBS
Is that hydroboost?

Yes. Instead of vacuum for brake boost, it uses pressure from the power steering pump.

Get your PS pump checked.

 

[Fitz98]

Went to check it out in person, it is hydroboost.
They said they did some brake work and replaced some calipers and lines and master cylinder. I imagine hydroboost portion would be independent of brake hydraulics and should not have been affected by the brake work. Am I thinking correctly here?
The PS pump does not make any unusual noises or show any signs of leaking FWIW.

 

[mk378]

Rather common for a PS pump to fail to provide steady pressure like it should. This will also affect the steering in a dangerous way.

 

[Astro14]

Went to check it out in person, it is hydroboost.
They said they did some brake work and replaced some calipers and lines and master cylinder. I imagine hydroboost portion would be independent of brake hydraulics and should not have been affected by the brake work. Am I thinking correctly here?
The PS pump does not make any unusual noises or show any signs of leaking FWIW.

Yeah, I don’t think the brake work caused anything. This is a power steering pump output pressure problem.