1990 1 ton Chevy. Lets call it my tow rig.:D Sliding calipers and Hydro boost.

This am both front calipers are locked after about 20 miles of driving. I bleed the pressure off at the MC and both release. Has done this one other time a few days after a NEW MC was installed. I rebuilt the caliper of the hottest rotor and had no issues till this morning.


I am having a problem blaming the MC as it only pushes fluid. IIRC there arent any checks in a MC that would cause this problem is there??

I shot both rotors with my IR gun and the passenger side was 280* and the drivers side was 204* so I may still have problems in the passenger side?? Passenger side flex hose was replaced less than a year ago. I have seen one caliper lock up due to various things, but never both. The MC or the Prop valve is the only common denominator in the system isnt it?


Will one sticking caliper cause pressure to build up so as to lock the other one?

I'm at a loss!!

What say ye??

Just off a hunch I loosened the bolts holding the MC to the Hydroboost assy and the MC kept pushing on the bolts until there was over an 1/8" gap between the two mounting flanges.

I pushed the MC back up against the HB and sure enough there wasnt any slack between the two. I took the MC off and looked and the pushrod recess in the piston and it wasnt anywhere close to the same shape as the HB push rod.

I spaced the MC away from the HB with a couple of thick body shims and plan to drive it this pm.

I'll report back my findings so maybe the next poor soul that has this issue won't loose as much sleep as I have over the last couple days.

What has happened to automotive parts suppliers in the last couple years? When I spend the extra money for something that is new instead of rebuilt is it to much to ask for it to be the correct one and work as it should?? :mad3:

To much MC pre-load. Your "spacers" should cure the problem.

dont run it with spacers. adjust the pushrod to the proper length.
if its not adjustable, its the wrong MC.

if it IS adjustable...who adjusted it? its not meant to be adjusted as needed. its a 1 time thing.

i know im still a noob..but why is this in the IH section?

dont run it with spacers. adjust the pushrod to the proper length.
if its not adjustable, its the wrong MC.

if it IS adjustable...who adjusted it? its not meant to be adjusted as needed. its a 1 time thing.

i know im still a noob..but why is this in the IH section?

Thanks for the info!! Its a Hydroboost system with a set pedal rod so no adjustment is avaliable. I have no plans to leave the washers, was only using them as a diagnostic aid. I have put one rebuilt and one new MC on this rig so I need some ground to stand on when I go for the third MC. I may see if GM can get it right this time??

I placed this here as the IH crowd are some of the most amazing techs/problem solvers in the game!!

Its not an IH product, but its one day gonna give its life so my sickly 800 can live a long life of wheelin.:D

Plus its another issue that someone else might encounter when making the HB conversion on their IH product.

Mods can move it if need be.:)