Confirm my master cylinder diagnosis?

[ASJT3]

Hey guys,

I have come to the conclusion that the master cylinder on my 99 Burban K1500 Vortec 350 is bad. It has very weak rear brakes. So weak in fact that they are seemingly non-existent; I can push the pedal down and inch or two with no apparent stopping power whatsoever. I've replaced the rear shoes, clean and oiled the star wheel for the adjuster, and also used a Motive Power brake bleeder to flush out old fluid and any air. Still, rear brakes are gutless.

I believe that the master cylinder has gone bad, specifically, fluid is probably getting past the piston that pressurizes the rear circuit. Do you agree?

[j cAT]

Originally Posted by ASJT3

Hey guys,

I have come to the conclusion that the master cylinder on my 99 Burban K1500 Vortec 350 is bad. It has very weak rear brakes. So weak in fact that they are seemingly non-existent; I can push the pedal down and inch or two with no apparent stopping power whatsoever. I've replaced the rear shoes, clean and oiled the star wheel for the adjuster, and also used a Motive Power brake bleeder to flush out old fluid and any air. Still, rear brakes are gutless.

I believe that the master cylinder has gone bad, specifically, fluid is probably getting past the piston that pressurizes the rear circuit. Do you agree?

I do not agree ! the master will not cause the rear drums not to brake at the proper rate.

with rear drums made in todays china quality world and the loss of asbestos as a shoe braking material the drums will need replacing at most often every shoe change. this depends on the quality of the shoes material. some times 2 shoe replacements you can get out of the drums.
The shoes if they wear mostly or only at the top of the shoes is saying the drum is too worn. the radius of the shoes is now not the same as the drum since the drum is too big.

be careful when you do this change testing the brake effort. have seat belt on.

[laxman21]

Originally Posted by ASJT3

Hey guys,

I have come to the conclusion that the master cylinder on my 99 Burban K1500 Vortec 350 is bad. It has very weak rear brakes. So weak in fact that they are seemingly non-existent; I can push the pedal down and inch or two with no apparent stopping power whatsoever. I've replaced the rear shoes, clean and oiled the star wheel for the adjuster, and also used a Motive Power brake bleeder to flush out old fluid and any air. Still, rear brakes are gutless.

I believe that the master cylinder has gone bad, specifically, fluid is probably getting past the piston that pressurizes the rear circuit. Do you agree?

Here is a mod to go from a squishy brake pedal to a firm brake pedal.

http://www.gmt400.com/forum/showthre...+cylinder+swap

[j cAT]

Originally Posted by laxman21

Here is a mod to go from a squishy brake pedal to a firm brake pedal.

http://www.gmt400.com/forum/showthre...+cylinder+swap

my experience with a soft pedal is some air in the system and also worn rotors/drums/braking material. as the brake cylinder/calipers have to receive more fluid to apply the proper brake effort this fluid reacts like a sponge. more fluid more soft pedal.

with all these worn items replaced the brake pedal is firm and travels less to give the expected braking effort.

[laxman21]

Originally Posted by j cAT

my experience with a soft pedal is some air in the system and also worn rotors/drums/braking material. as the brake cylinder/calipers have to receive more fluid to apply the proper brake effort this fluid reacts like a sponge. more fluid more soft pedal.

with all these worn items replaced the brake pedal is firm and travels less to give the expected braking effort.

Seems to be a common problem with this version of suburban. I've done all of them, what you stated, at the same time, power bleed the system, same spongy pedal and long travel. Since brake fluid is not appreciably compressible, the small amount of extra fluid in the calipers shouldn't have any effect on the pedal. When I do this mod I will also be doing braided lines as there are posts about improvements with them too.

[j cAT]

Originally Posted by laxman21

Seems to be a common problem with this version of suburban. I've done all of them, what you stated, at the same time, power bleed the system, same spongy pedal and long travel. Since brake fluid is not appreciably compressible, the small amount of extra fluid in the calipers shouldn't have any effect on the pedal. When I do this mod I will also be doing braided lines as there are posts about improvements with them too.

you are correct the extra fluid with worn braking components should not be that great to affect the pedal but it does. when you just give this vehicle a good sized bleed about 1 qt total in a vehicle this old that alone reduces the sponge effect. this is because most do not properly handle/bleed the braking system.

when most all working the braking system they push back the calipers ..when this is done with the bleed screw closed all that dirty contaminated fluid works back into the brake system and the ABS valve body . then they give the fluid a quick squirt and call it good .

I open the bleed screw , then push back . then you see in the container all what I am talking about along with the off color fluid.

so just the proper handling of the fluid makes a difference now you throw in a worn brake system rotors/drums / braking material. it adds up to an accident in the making.

then you get into the brake lines/hoses most do not know anything about this . at this age these hoses are damaged from years of contaminated fluid.

[DeltaP]

Totally agree with j cAT. One of the differences between a good brake job and a shade tree, pad slapped brake job.

[ASJT3]

To answer your comment about bleeding, I completely flushed the system with the powerbleeder (i.e., removed all old fluid from res, and bled all wheels, starting at passenger rear, until only clear fluid was flowing out of each wheel). Also, haven't done any caliper work (pads are nice and thick).

I did go ahead and replace the drums this weekend. I will say that braking is better, but the pedal still feels spongy, no where near as firm as the pedal in the '93 K1500 or in the '96 Buick Roadmaster.

I'm thinking this still has to be the MC, and I'll tell you why. I worked on the Buick's brakes this wknd (new drums also, bleed system) and with the car off, I could pump up the brakes to the point where I could not physically push the pedal down without feeling like i was going to bend the pedal arm. Not so with the suburban; it takes some effort, but I can slowly but surely completely push the pedal down.

The mod that laxman posted is interesting, I'm wondering if it's worth it if I'm going to replace my old MC anyway, to do it with a GMT800 MC. I had also bought a rebuild kit for the Buick MC, which will fit the GMT400 MC apparently, so that's another option.

[j cAT]

Originally Posted by ASJT3

To answer your comment about bleeding, I completely flushed the system with the powerbleeder (i.e., removed all old fluid from res, and bled all wheels, starting at passenger rear, until only clear fluid was flowing out of each wheel). Also, haven't done any caliper work (pads are nice and thick).

I did go ahead and replace the drums this weekend. I will say that braking is better, but the pedal still feels spongy, no where near as firm as the pedal in the '93 K1500 or in the '96 Buick Roadmaster.

I'm thinking this still has to be the MC, and I'll tell you why. I worked on the Buick's brakes this wknd (new drums also, bleed system) and with the car off, I could pump up the brakes to the point where I could not physically push the pedal down without feeling like i was going to bend the pedal arm. Not so with the suburban; it takes some effort, but I can slowly but surely completely push the pedal down.

The mod that laxman posted is interesting, I'm wondering if it's worth it if I'm going to replace my old MC anyway, to do it with a GMT800 MC. I had also bought a rebuild kit for the Buick MC, which will fit the GMT400 MC apparently, so that's another option.

so the vehicle brakes better now with the new drums and shoes ? now you feel the pedal is spongy .could be the front calipers are sticking or the pads fit is too tight. make sure the pads are not stuck.

open the bleed screw and using a C clamp push back the cal piston see that the pads are not stuck [using your fingers no tools] also check that the piston is not binding. when you open the bleed screw you are dumping out all the fluid trapped in the cal piston . see if the debris is in your recovery container .

make sure the brake hoses are working properly ..

with that done and you still have a soft pedal it is possible air is trapped in the ABS valve body.

[ASJT3]

Originally Posted by j cAT

so the vehicle brakes better now with the new drums and shoes ? now you feel the pedal is spongy .could be the front calipers are sticking or the pads fit is too tight. make sure the pads are not stuck.

open the bleed screw and using a C clamp push back the cal piston see that the pads are not stuck [using your fingers no tools] also check that the piston is not binding. when you open the bleed screw you are dumping out all the fluid trapped in the cal piston . see if the debris is in your recovery container .

make sure the brake hoses are working properly ..

with that done and you still have a soft pedal it is possible air is trapped in the ABS valve body.

Gotcha, i'll give it a try and report back.

[j cAT]

Originally Posted by ASJT3

Gotcha, i'll give it a try and report back.

so where are you with the spongy pedal ?

[ASJT3]

Originally Posted by j cAT

so where are you with the spongy pedal ?

Still soft..haven't had time to get to it yet. Had to replace front brakes on the Buick and been busy with outdoor activities. Maybe I'll get to it tonight; I'll be sure to post back!

[j cAT]

Originally Posted by ASJT3

Still soft..haven't had time to get to it yet. Had to replace front brakes on the Buick and been busy with outdoor activities. Maybe I'll get to it tonight; I'll be sure to post back!

so its not that bad now ?

[ASJT3]

Originally Posted by j cAT

so its not that bad now ?

I used a c-clamp to get the old fluid out of the calipers. It was pretty dirty on on pass side with some debris, and relatively clean on driver's side.

Brake feel might be slightly better, but is largely unchanged. It does feel like there is air in the system, which is weird because I pressure-bled the system. Would trapped air in the ABS system remain even after powerbleeding?

I'm wondering if it's worth replacing the calipers, too..

[j cAT]

Originally Posted by ASJT3

I used a c-clamp to get the old fluid out of the calipers. It was pretty dirty on on pass side with some debris, and relatively clean on driver's side.

Brake feel might be slightly better, but is largely unchanged. It does feel like there is air in the system, which is weird because I pressure-bled the system. Would trapped air in the ABS system remain even after powerbleeding?

I'm wondering if it's worth replacing the calipers, too..

that is the problem of air trapped in the ABS valve body ...what some have done was get the ABS to activate and then bleed again. this has worked for some.

the calipers with the procedure of the C clamp and the bleed screw open should have revealed any caliper issues. the debris you saw is why I do it this way . pushing back the caliper with worn pads that crap gets into the ABS..

dealer ship has the ABS bleed computer controller to do this ..perhaps a repair shop has this also..