1994 Chevrolet dually brake ??

[turbo-power]

I have been experiencing a soft/spongy brake pedal and after a period of stops or hard brake pedal effort the brake light will come on. It feels like the master cylinder is failing but there is no sign of fluid leakage like I have seen on other vehicles I have owned. Is it possible for the master cylinder to fail and not have fluid leak out the back of it ? This is on my 94 dually. Thanks for any tips

[lx22f/c]

Absolutely. The seals bypass internally. Good test is to put the truck in park with it running and apply light but constant pressure on the pedal. Usually it will fall away slowly. If it does replace it and i would go with a new one over a reman. Hope this helps.

[herewego]

Have also seen this,in my years of auto repair,your rubber brake hoses or just one hose may be seperated,on the inside .. agree with post # 2 also ...

[Seann45]

When was the last time you changed brake fluid? lets go with the easy fixes first. Then old rotting hoses which you would notice while changing brake fluid.. then wheel cylinders or master cylinder

[WVDusty]

Depending on when or if ever you have replace the rubber lines. That would be my first guess. Like hearwego said. I also have had this happen with the same symptoms you are describing.

[turbo-power]

Thanks for the replies.

I am going to look back at it today and check for swelling on the rubber hoses and if none found then I will put on a master cylinder.

[WVDusty]

The rubber lines may not show any abnormalities on the outside. They typically will deteriorate on the inside.

[etex211]

Does this truck have a hydraboost unit?

[turbo-power]

Originally Posted by etex211

Does this truck have a hydraboost unit?

Yes . But it has been replaced last week because the other one was leaking and the spongy feel started before the hydroboost started leaking and is still spongy.

[turbo-power]

Ok , I had the DW push on the brake pedal and hold it and I checked the 2 front hoses and the 1 rear hose and I did not see any swelling or ballooning while under pressure. So I replaced the master cylinder and that did not correct the problem either.
So back to square one. Should I replace the hoses ? If that does not fix it then what ? There is no brake fluid leaking anywhere its just not holding pressure. Thanks for any advice.

[turbo-power]

Thought I would add this info also.

While in park you can press the brake pedal and hold it and as the pedal goes down due to not holding under pressure the brake light comes on.

The brake fluid leaves the master cylinder goes thru a proportioning valve then the fluid for the front goes to the calipers and the fluid for the rear goes thru a RWAL module then to rear wheel cylinders.

Since the brake light is coming on could there be something going wrong with the RWAL ? Not sure how that works or what is triggering the light to come on.

[etex211]

Originally Posted by turbo-power

Ok , I had the DW push on the brake pedal and hold it and I checked the 2 front hoses and the 1 rear hose and I did not see any swelling or ballooning while under pressure. So I replaced the master cylinder and that did not correct the problem either.
So back to square one. Should I replace the hoses ? If that does not fix it then what ? There is no brake fluid leaking anywhere its just not holding pressure. Thanks for any advice.

I believe the problem with the brake hoses on those Chevy trucks is that they swell on the inside. It's a common problem on the Chevy trucks from that era. I had to replace the front hoses on a '92 half ton truck that I had. The symptom on that track was that the caliper would not release.

Have you replaced all of the fluid in the system? I went to a seminar given by one of the brake parts manufacturers a few years ago, and one of the things they brought up is that the brake fluid on that era of GM truck needs to be replaced from time to time to prevent damage to the antilock unit by the fluid.

While we're on the subject of the antilock unit, it is worth noting that when you press the peddle, you are not really in direct contact with the system. The antilock unit controls what is applied to the system.

Hope this info helps.

[turbo-power]

I have not yet bleed the system completely yet . That's the next thing I will do before replacing anything else and make sure there is no air in the system and all fresh fluid.

[Paintinfool3]

I am not sure if you have checked pads and shoes ... I had my right rear shoe shot and mine were spongy because it was over extending the cylinder on that wheel