Spongey Brakes

[Nate52]

This is my first post - sorry if this has been covered in another thread.

I have an ‘80 K5 I’m working on and brakes are failing. The brake pedal is soft and the truck only slows down mildly with the pedal to the floor. The brakes worked fine a few months ago. Over the course of just a couple weekend rides to town they became so soft that it’s unsafe.

There are no obvious leaks (doesn’t mean it isn’t leaking, but it doesn’t look like I lost all my fluid at once.) there is fluid in the large side of the master cylinder but none in the small side. When I depress the pedal I can see the brake booster moving but the fluid level in the master cylinder doesn’t move at all when the pedal is pressed. I don’t have much experience when it comes to brakes so any troubleshooting advice would be great. Thanks in advance.

 

[Bent77]

Ok, lets get some brake fluid in both sides of the master cylinder first, then bleed the brakes.
There may be a leak, and we need fluid in there to find it

 

[Bent77]

Here's a quicky video on bleeding the brakes
Yours is not ABS, so start at the rear brakes. Make sure to refill that master several times as you go

 

[frankin5]

With the rear empty the proportioning valve will have activated. It will have to be reset and that is where it is nice to have the trick tool to keep it center while you bleed the brakes.

 

[rampage]

I’ll bet one of your rear brake cylinders is leaking and if you pull your drums off you may find it or you may even see the splatter on the inside of the wheel.

 

[wasted wages]

First thing I would do, is a visual inspection of the entire brake system.

look over all of the brake lines both flexible and the hard lines... look over each wheel (back side)
to see if you find any fluid on the wheels or tires.

Pull back the carpet on the driver side firewall under the dash and see if there is any sign of fluid leaks running down the firewall below the brake pushrod.

If you have a vacuum booster you may see fluid drips under the master cylinder where it bolts to the booster.

A lot of times leaks make themselves known
by dripping fluid from a wheel, wet hoses or wet fittings.

 

[Nate52]

Frankin - is this the tool? My brake light is on so your probably right about the proportioning valve being closed to the rear. https://www.jegs.com/i/JEGS/555/632...MImaTThLn_6gIVE77ACh1rUwzqEAQYAiABEgKC2_D_BwE

Y’all are probably right about it being a leak. I wasn’t 100% sure if the small section in the Master cylinder was for the rear brakes. The inside of the cap had the front section labeled as “fluid side” so I second guessed myself a little bit.

Hopefully I’ll have a little time this weekend to bleed the brakes.

 

[Wes Harden]

Disc brakes use more fluid than drums. Also as the pads where the piston doesn't return, more fluid is required to fill the area.
Drum brake wheel cylinders return the fluid to the master after pedal release. Only require more fluid to apply as shoes wear.

After you fix the leak in rear? Top off master, open left front bleeder and apply brake pedal slowly but firm till stops. If combination valve is clean and not rusty inside, it will recenter

 

[rampage]

Y’all are probably right about it being a leak.

Since the brake system is a closed circuit then any loss of fluid has to be a leak.

 

[frankin5]

That be the one.

 

[Nate52]

Bled the brakes - brake light is now off on my dash and the brakes work just as well as they ever have. I took it for a test drive and drove it fairly hard. When I got back I checked the brake fluid level and it appeared unchanged. I guess I’ll have to monitor the fluid level occasionally to check for a slow leak. I Appreciate the help from everyone.