When engine on pedal sinks to floor with minimal braking power

I would suspect there is air in the lines. These guides can help you fix it.

https://youtu.be/w7gUsj2us0U

and

https://youtu.be/WDxvEQrMkBg

and

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please run down these guides and report back.

You will need a scanner to open the ABS valve to get the air out.

Spongy but has a pedal? What condition are the brake hoses in? Might have one that is swelling giving you a bad pedal feel?

Excellent addition to this thread! Please feel free to help out whenever you are on the site :) Cheers, Ken

Try power bleeding the system rental shops carry the tool it may be an abs problem however and you'll need a pro to do that if it is.

Is this a pickup? It should have abs if it is. Rear brake pressure won't be a lot as most stopping is done on fronts with discs. We don't have specs on that. That is why I said try a power bleeder.

Vern:
It sounds like a bad master cylender. When you say there are no front brakes, do they work at all? Are the calipers ok? Finally, do the rear brakes work properly?

Joe

Vern:
My guess is that you got a bad master cylender. I can't think of anything that would send the fluid back into the resevoir.

I'll ask a couple of my buddies on this web site to take a look, but I'm not sure they will know what to say.

Let me know what they recommend.

Joe

Does this master cylender have the abs attached to it or is the abs on its own this is the most common used This will cause the issue you are having

EHCU Valve

See Figure 1

The EHCU valve is mounted near or under the master cylinder and combination valve. The valve is not serviceable and must be replaced if malfunctioning.

Fig. 1: Exploded view of the Electro-Hydraulic Control Unit (EHCU)
Front Wheel Speed Sensors

With one exception, on both 2WD and 4WD vehicles, the front wheel speed sensors are permanently mounted to the brake rotor splash shield. With the exception of the 1992-98 Suburban, if the sensor fails the rotor and splash shield must be removed. On 4WD vehicles, the hub and bearing assembly must also be removed for access.

The front wheel speed sensors on 1992-98 4WD Suburban vehicles are removable without disassembly of the hub or brakes.

Rear Wheel Speed Sensors

Except for 1992-93 Suburban, the rear wheel speed sensors are held by 2 bolts at each rear wheel. The brake drum and primary brake shoe must be removed for access.

The 1992-93 Suburban receives the rear wheel speed signal from the Vehicle Speed Sensor (VSS) buffer. The VSS is located at the left rear of the transmission. The buffer, a unit for interpreting the electrical signal from the sensor, is located behind the instrument cluster. The buffer, formerly known as the digital ratio adapter controller (DRAC), is matched to the final drive and tire size of each vehicle. If the final drive or tire size is changed, the buffer unit must be replaced to maintain accurate speedometer/odometer readings and proper ABS function.

Here i another system they used
C/K Series

MANUAL BRAKES

See Figure 2

Disconnect the brake lines at the valve. Plug or cap the lines and ports.
Unplug the switch wiring connector.
Remove the valve-to-bracket bolts.
To install:

Install the valve. Tighten the bolts to 20 ft. lbs. (27 Nm).
Bleed the system.

Fig. 2: Combination valve-C/K Series
POWER BRAKES

Disconnect the brake lines at the switch. Plug or cap the lines and ports.
Unplug the warning switch wiring connector.
Remove the anti-lock brake system control module from the bracket. See the procedure later in this section.
Remove the bolts attaching the ABS isolation/dump valve to the bracket. See the procedure later in this section.
Remove the nuts that attach the master cylinder and bracket to the booster.
Remove the bracket and combination valve assembly.
To install:

Position the bracket/valve assembly and install the master cylinder-to-booster nuts. Tighten the nuts to 20 ft. lbs. (27 Nm).
Install the ABS isolation/dump valve nuts. Tighten them to 21 ft. lbs. (24 Nm).
Install the ABS control module.
Connect the wiring.
Connect the hydraulic lines.
Bleed the system.
SWITCH CENTERING

Whenever work on the brake system is done it is possible that the brake warning light will come on and refuse to go off when the work is finished. In this event, the switch must be centered.

Raise and support the truck.
Attach a bleeder hose to the rear brake bleed screw and immerse the other end of the hose in a jar of clean brake fluid.
Be sure that the master cylinder is full.
When bleeding the brakes, the pin in the end of the metering portion of the combination valve must be held in the open position (with the tool described in the brake bleeding section installed under the pin mounting bolt). Be sure to tighten the bolt after removing the tool.
Turn the ignition key ON. Open the bleed screw while an assistant applies heavy pressure on the brake pedal. The warning lamp should light. Close the bleed screw before the helper releases the pedal.
To reset the switch, apply heavy pressure to the pedal. This will apply hydraulic pressure to the switch which will re-center it.
Repeat Step 5 for the front bleed screw.
Turn the ignition OFF and lower the truck.
If the warning lamp does not light during Step 5, the switch is defective and must be replaced.

Hi,

This is a common problem with these trucks you must get a ABS scanner that will open the ABS valves to let the air out.

Here is a video to show you how to connect.

https://youtu.be/rTtAnsOlZU4

If that does work I would get hold of three vise grips and clamp off all the flexible brake hoses, see how the pedal is then. if it is firm the problem will be in one of the brakes if it sinks it is the master cylinder.

Check out this guide.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Mark (mhpautos)