Soft Brakes. Big Estimate. New Brake fluid and now fixed?

[Spool]

Hello.

My name is Thomas. I'm new here with a 73 C20 Chevy living in Los Angeles.

I've been working on the truck for about 2 years now but still have much to learn.

About 5 weeks ago, the brakes went soft.
I took it in to what I thought was a trusted mechanic.
He tells me this, that, brake booster, bad lines, etc.
$150 just to look at it plus $1000 to replace everything.

I have him bleed the brakes and replace the fluid to see what happens and now they are fine.
They have actually been fine since then. (5 weeks)

But i'm curious. What led to the problem? Why did they go soft and what should I be looking for as the culprit?
I replaced the brake pads about a year ago and just before I'm coming to a complete stop they will squeal a bit.
The brake lines look fine. Should I be looking at my brake booster or master cylinder and if so, what should I look for?

So something needs to be replaced but I'd like to ask here rather than go to the mercy of some mechanic as I am not familiar with brake systems.

Thanks in advance!
-Thomas.

 

[Georgeb]

For now I would keep an eye on the fluid level and look for wet spots under the truck and by the calipers and wheel cylinders. Perhaps you have small leak. Otherwise a flakey master cylinder could be the cause.

 

[Spool]

Thank you.

I just read/watched some stuff on line as well.

I was thinking tomorrow I'll have the brake pedal pressed to see if I can spot a leak anywhere along the lines or wheels.
If I don't see a leak anywhere then perhaps it would be the master?

 

[highdesertrange]

do a test. check master level, with the truck running in park step on the brake pedal as hard as you can, keep the pressure on the pedal as long as you can. see if the pedal slowly moves down. if your pedal drops recheck your master. if the fluid is down look for a leak(you have one). if fluid is the same you have a bad master. highdesertranger

 

[4WDKC]

Brake fluid attracts moisture, so over time moisture gets into the brake fluid. When that happens the brake pedal can go soft due to the water being compressible unlike brake fluid this is also why steel brake lines rust from the inside out and why manufactures now recommend having brake fluid flushed in new cars as part of their maintenance.

 

[chengny]

If the brakes suddenly go soft and then can be completely revived with just a change/flush and bleed of the fluid, it would indicate a MC issue.

Sometimes as a cup seal gets old, it starts to lose it's flexibility. It's lips are supposed to be pressed up against the the cylinder bore of the MC as the piston moves it forward. On the return stroke that outward pressure is released and the outer lip backs off from the bore wall.

This happens thousands and thousands of times without fail. Then one time, on the return stroke, the lips release as always. But don't have the same natural resiliency as when they were new and, at the end of the stroke, they don't spring back out against the bore.

So, the next time the brakes are applied and the piston moves forward, that pressurized outward force against the bore doesn't develop. The seal slips right down the bore without compressing the fluid in the cylinder.

Since the compression required to pump the brake fluid isn't developed in that cylinder (there are two pistons/cylinder bores), no pressure is applied to the brakes in that hydraulic circuit. Consequently, you are then braking at 50% capacity.

Water is not compressible, but it does present another problem, when heated it turns into steam - which is highly compressible. When allowed to accumulate in the brake fluid, the water settles out in the lowest points. The lowest points are at the wheel cylinders and calipers. Those areas also happen to be the hottest parts in the system. During heavy braking, the heat generated by the friction of the pads/shoes against the rotors/drums, is transmitted to the water in the brake lines. If the water gets hot enough, it flashes off into steam. Steam is no more compressible than air. So now, when the brakes are applied the steam just compresses and does not cause the pistons in the wheel cylinders/calipers to move. Braking capacity is greatly reduced - until the steam cools and condenses back into water.

You didn't note any loss of fluid so there is probably not any leakage of sufficient capacity that would reduce braking capacity. Plus you would see brake fluid on the driveway.

And as far as air in the system - that is generally a symptom of a leak. Air usually only enters the hydraulic system if the level in the MC drops to a point low enough that the piston can suck it in from atmosphere and pump it into the piping. Unless you had the brake lines open for repair the day before this started, it wasn't an air issue.

The flushing out of old brake fluid/addition of fresh brake fluid - along with the vigorous stroking of the MC piston during the bleeding procedure, probably freed up the slack cup seal in the master. I wouldn't do anything other than watch your MC level and add fluid if required. There is a good chance your brakes are okay now. Maybe not for ever, but at least for awhile.

 

[75gmck25]

I've had master cylinders go bad a couple of times shortly after replacing brake pads, and i think it may have been my bleeding technique that caused the problem.

When I bled the brakes after installing new pads I pushed the pedal to floor several times as far as it would go. My guess is that this pushed the rubber seal inside the master cylinder past a ridge that had formed as the brakes were used over several years. When the seal scraped across that ridge of built up dirt/grime, it damaged the seal slightly and it started to deteriorate. That accelerated the failure of the master cylinder, and within a month or two I had to replace it.

Bruce

 

[Camar068]

Old post but I'll add as well.

if you've got a fluid leak, your going to see it on the lines or components. Look for clean or wet spots on brake components/lines or fluid on nearby stuff.