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**hatzie** · 12-17-2021, 08:32 PM

T400 owners upgrade to the T800 series master cylinder. This is a problem with your truck.
Sounds like you may have air trapped somewhere in the system.

Did you bench bleed the master cylinder to push all the air out before bleeding? The brake pedal will not push the piston to full depth to eliminate air pockets in a new master cylinder.
I use a pressure pot bleeder to push fluid through the system. This helps a great deal on vehicles that use the ABS pump to meter and proportion the brakes.
You can DIY one or buy one. At this point the Motive Products bleeder is about the same price as the parts to build one.

What are you using for power steering fluid?
Did you bleed the power steering and hydroboost system?

Another thing to look into is your tire size. If you're running larger than stock tires and haven't re-coded the ABS computer for the larger tires it can make the brake pedal feel somewhat odd.

12-17-2021, 08:32 PM	#2
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,916	Re: '01 2500 HD spongy brake pedal T400 owners upgrade to the T800 series master cylinder. This is a problem with your truck. Sounds like you may have air trapped somewhere in the system. Did you bench bleed the master cylinder to push all the air out before bleeding? The brake pedal will not push the piston to full depth to eliminate air pockets in a new master cylinder. I use a pressure pot bleeder to push fluid through the system. This helps a great deal on vehicles that use the ABS pump to meter and proportion the brakes. You can DIY one or buy one. At this point the Motive Products bleeder is about the same price as the parts to build one. What are you using for power steering fluid? Did you bleed the power steering and hydroboost system? Another thing to look into is your tire size. If you're running larger than stock tires and haven't re-coded the ABS computer for the larger tires it can make the brake pedal feel somewhat odd. __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful. Last edited by hatzie; 12-17-2021 at 08:54 PM.