dragging brakes?

[VPRob]

My VP has VT front brakes with slotted rotors and VT master and booster. Upgrade has been in the car for a few years. In the last few days I have experienced a disturbing strong vibration or shudder for a second or 2 when accelerating after releasing the brake pedal. This has happened 3 times. It feels like it is shaking the whole car and then stops. I am interpreting it as a brake problem.
Front of car is currently lifted and wheels off. Rotating the front wheels it is clear that the driver's side wheel is dragging and feels worse on some areas of the rotation. I have been experiencing some vibration under braking recently and the whirring from the (straight) slotted rotors has seemed to be getting more noticeable. Stopping power has still been good but all 4 rotors were hot after a relatively short drive yesterday.
I'm pretty sure I've got free play in the push rod but that's an obvious candidate.
Any tips on how to proceed?

 

[Davo_vx]

I had a similar problem on my vx that I fixed by replacing the master.

 

[_R_J_K_]

feels worse on some areas of the rotation

Based on that and the shuddering you describe the first place I would go I would be looking at would be the rotor to see if it's warped. I know you say it's straight but I reckon the slotting would amplify it significantly if it were even slightly bent.

Has a bleed done anything? Z-bar bushes up to scratch too? Uneven/slanted wear on the pads?

 

[VPRob]

My next step is to rotate the rotor up to have a look at the pads. With a bit of gentle levering with a screwdriver the pads retract further and the rotors rotate freely. That seems to indicate the pistons are free in the calipers and the pins are free also. When I run the motor to charge up the vacuum assist the pads stay retracted and the rotors are free but when I apply the brake pedal and release it the driver's side will rotate but with a noticeable amount of drag compared to the passenger side.

 

[VT2Commie]

Please check your drive shaft centre bearing carrier rubber, disturbing vibration /shudder under acceleration for a short second is drive shaft related.

As for the brakes, when applying them at speed, the caliper slide pins may feel/look like they are free but I would remove, clean and relube them and make sure the rubber guide pin boots are not torn or split.
Do a brake fluid flush by bleeding them. With the correct fluid type of course.

And go get the brake rotors machined or replaced if under spec.

 

[VPRob]

Thanks for the responses everyone. I will check my centre bearing but I had dismissed that as I completely rebuilt the driveshaft about 5 years ago.
The Gregorys flags a blocked or partially blocked port in the MC so I am thinking a bleed is a good way to go.

 

[vc commodore]

My next step is to rotate the rotor up to have a look at the pads. With a bit of gentle levering with a screwdriver the pads retract further and the rotors rotate freely. That seems to indicate the pistons are free in the calipers and the pins are free also. When I run the motor to charge up the vacuum assist the pads stay retracted and the rotors are free but when I apply the brake pedal and release it the driver's side will rotate but with a noticeable amount of drag compared to the passenger side.

Does sound very much like the caliper pins are starting to seize on the right hand side....I'd pull them apart, clean the pins and pin slot up, re-grease them and see how that goes.

I'd also check the rotor isn't warped, as a result Good chance it will be if the brakes have been dragging on them....

 

[VPRob]

what's the appropriate grease to use on the caliper pins?

 

[Davo_vx]

I use the bendix high performance synthetic stuff.

 

[losh1971]

Yeah Synthetic caliper slide grease Rob. I reckon Repco now stock it. I normally buy it from eBay. I buy a tube once every 8 - 10 years. The Bendix satchet stuff is good but might need to get it from a brake place.