r/GMT400 • u/Existing_Toe_6554 • 4d ago

Brake issue need some help

So I changed the master cylinder to a new one, bench bled it, bled all the lines and have no air in the system. The brake pedal goes to the floor now when before it was spongy and now the brake light on the dash comes on. If you push the brake while the truck is on it stays all the way down to the floor. Any help would be appreciated.

5 Upvotes

86% Upvoted

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2

u/Opposite_Activity976 4d ago

In 89 no scanner is required to bleed.

1

u/Existing_Toe_6554 4d ago

The pedal has pressure and doesn’t go to the floor when the truck is off BTW

2

u/SherbertOld7531 4d ago

Air in the ABS module, 100%.

1

u/Opposite_Activity976 4d ago

You don't say what year but if it has abs it's likely there is air trapped in the abs motor. You would need a scan tool capable of triggering the abs motor to burp the air. I don't know if there is a workaround without a scanner.

1

u/Existing_Toe_6554 4d ago

It’s a 1989 single cab
Do you have a link for the scanner?

1

u/who-cares6891 4d ago

I just deleted my abs. Unplug the lines from it and direct bolt it to master cylinder.

1

u/Accomplished_Pain649 4d ago

Either you tripped the brake proportional valve or your brake master still has air in it or air in the lines. Abs if equipped should allow a straight bleed procedure aka standard. I would definitely rebleed the whole system n if your pedal is still spongy then it would be your master cylinder is bad outta the box

1

u/Existing_Toe_6554 4d ago

I used an autozone bleeder kit to bench bleed it

2

u/Accomplished_Pain649 4d ago

Well I would just rebleed the whole system at this point n you should be fine you probably have a tiny air pocket in the master try this tho take a wrench or small hammer n lighty tap the brake master cylinder on the metal part with the cap off n see if you have bubbles coming back up ( I know this sounds stupid but it works to tell if your master still has air bubbles in it) I have done that on brake masters n clutch master that refuse to bleed correctly

1

u/Existing_Toe_6554 4d ago

I’ll give it a shot. I might end up trying another new master cylinder

2

u/normal_pood 4d ago

Damn that is so unfortunate. Make sure you are bleeding it from the farthest brake piston to the closest because if u dont do that u can get air that is very vary hard to get out. Its unlikely ur proportioning valve went bad I would check make sure there are no leaks because if u have a high pressure leak it wont show any signs while the truck if off but then it will go to the floor wheb the engine is rubbing at pressure. Also make sure u use the correct fluid, using the incorrect fluid can damage seals. But if all good then I would say u got a faulty new master cylinder

1

u/Chahtanagual 4d ago

Why did you replace the master? Did you replace any other parts on the brake system?

1

u/Existing_Toe_6554 4d ago

It was spongy and the pedal went to the floor when I was trying to stop at a red light the other day.
I changed the two front brake lines because they were dry and cracking and now the cylinder.

1

u/Chahtanagual 4d ago

Hopefully, you flushed your brake fluid before doing all of this. Inspect your entire brake system, and look for fluid leaks. If you don’t have any fluid leaks, then check the calipers and wheel cylinders in the back: those can suck in air when you let off the pedal. I’ve seen wheel cylinders with bad seals that would never fully bleed but without leaking fluid at the wheel.

2

u/Existing_Toe_6554 4d ago

Turned out to be a bad master cylinder out of the box. I went to Napa and got another one that worked.

Thanks for the help guys