M1008 rear brakes

[Chevykev]

The rear brakes on my M1008 sometimes lock up as if the fronts are not working, doesn't do it all the time though, seems to right itself after a while, any ideas ?

 

[Keith_J]

Probably the ride height proportioning valve mounted on the rear axle and connected to the frame.

In proper operation, it reduces rear brake pressure when there is no load. Many remove this valve as problems are common. There was a GM Technical Service Bulletin covering this modification.

 

[GunnyM1009]

@Keith_J is onto something I would also check your soft lines. If they are old and worn they can start to collapse on themselves restricting fluid travel and causing issues. After that a good brake bleed at all four corners would also be advised.

 

[ezgn]

@Keith_J is onto something I would also check your soft lines. If they are old and worn they can start to collapse on themselves restricting fluid travel and causing issues. After that a good brake bleed at all four corners would also be advised.

I would pull your drums off, clean everything up on your braking components, check the proper fit of your brake shoes, and adjust the brake shoes the proper distance from your drums when re installing. It could be your shoes were not adjusting properly as they wear. For your safety and the safety of others, I would not drive the vehicle until it is working properly. You lose all your stopping power when your rear wheel locks up.

 

[cucvrus]

I would pull your drums off, clean everything up on your braking components, check the proper fit of your brake shoes, and adjust the brake shoes the proper distance from your drums when re installing. It could be your shoes were not adjusting properly as they wear. For your safety and the safety of others, I would not drive the vehicle until it is working properly. You lose all your stopping power when your rear wheel locks up.

Excellent advice. And if the brakes have oil on them they are junk. Change them and correct the oil leak. Oil leaking has always been the issue with the rear drum brakes on the M1008-M1031. Do it right and use new axle flange seals also.

 

[Chevykev]

Thanks for the replies, it seems the front half of the master cylinder had lost most of its fluid. Topped it up and it seems fine, but where did it go, no obvious leak. Was at an event at the time, so will check properly this weekend.

 

[cucvrus]

Thanks for the replies, it seems the front half of the master cylinder had lost most of its fluid. Topped it up and it seems fine, but where did it go, no obvious leak. Was at an event at the time, so will check properly this weekend.

With the additional information I am going to guess a wheel cylinder or a rusted thru brake line. If it is rusted thru start at the front and replace all of it. It is easier now than waiting for the next blow out.

 

[nyoffroad]

With the additional information I am going to guess a wheel cylinder or a rusted thru brake line. If it is rusted thru start at the front and replace all of it. It is easier now than waiting for the next blow out.

Also look under the driver side floor mat if you have them, a leaky master cylinder will sometimes leak inside the cab.

 

[Chevykev]

Thanks for the replies. Topped it up , used the truck, braked hard a few times, checked everything suggested, nothing, totally dry everywhere & brakes working fine ! Only possible leak was a bit wet at front of master cylinder cover .

 

[ezgn]

Thanks for the replies. Topped it up , used the truck, braked hard a few times, checked everything suggested, nothing, totally dry everywhere & brakes working fine ! Only possible leak was a bit wet at front of master cylinder cover .

You might need a new gasket under your master cylinder cap.

 

[Keith_J]

DOT5 silicone or DOT3 glycol? Mixing the two is very bad. DOT5 lasts much longer, doesn't absorb water, doesn't corrode, doesn't eat paint..

 

[87cr250r]

The trouble with DOT 5 fluids is that they don't absorb water. Any water that ends up in your system will end up in the calipers and cause seized pistons. There is a reason the low maintenance civi vehicles prefer not to use DOT 5 fluids.

And no, don't mix the two. My truck had mixed fluids in the front system and you could see it from the swelling of the gasket on the reservoir cover.