Brake pedal wants to sink if pressed harder when at a stop ...

Upgraded my brakes today (calipers etc) but allowed the master cylinder to go dry will putting on new parts. I didn't have anything to plug up the rubber brake lines with.

Drove home from where I did the work. Pedal travel is "further" than I'd like but not really different than how it was originally; it stops fine; better than before overall due to the new brakes and SS lines I installed.

But when I parked, I noticed I can push the pedal further yet. It just keeps going, although I didn't try to floor it.

Some notes:
Bleeding was done on the rears, then the brake proportion valve, then fronts, i.e. furthest-to nearest the master cylinder.

I used a clear hose; got new fluid through it, no bubbles. Check-valve (speed) bleeders are on the front and I looped the tube up and over for all brakes for the "one man" method, ensuring no air in the line. Used about a quart of new fluid; I was generous with pumping.

And I placed a wood block under the brake pedal while doing this, so that the pedal couldn't go all the way to the floor.

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So if I ran the master cylinder dry, didn't my bleed procedure "fix" that, regardless? The symptom as I've described it seems to point to a bad master cylinder, even though it didn't have this issue before I wrenched on the truck.

I will double-check tomorrow for any leaks, when I have time, and look at where the master cylinder reservoir level is again.

 

You likely still have air in the system/lines
or a faulty seal somewhere.

But a leaking seal would likely find hydraulic fluid
leaking out somewhere.
And the peddle would not return to it's up (neutral) position.

Does the brake peddle return to its up(neutral) position
immediately after releasing pedal pressure ?

Spongy brakes tend to lead to air being in the hydraulics.

What's the wood block under the pedal do - for ?

There could also be a vaccum leak with the brake booster.
Can you hear that engage as it builds up air pressure ?

 

I'd read that when bleeding brakes, you shouldn't allow the pedal to go all the way to the floor. Something about the master cylinder being extended past its normal range and hurting seals. So that's why I used the wood block,



The pedal returns to normal position just fine, even with repeated usage while sitting still. Builds pressure; it's just that if you're sitting there in Park, and press the pedal, you can press get pressure, and then continue to press and the pedal won't seem to stop.

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Drove it again, around town. I wouldn't characterize the brakes as spongy while driving. With all-new "Tundra" (231mm) calipers (NAPA Eclipse) / Toyota pads, shims and rotors, and Wheeler's SS front brake lines, the pedal feels somewhat "lighter" (a little less resistance?) and it's now very progressive in both applying and releasing the brakes.

***************
One thing I noticed when I got the truck last year is that the first inch of pedal travel induced a tiny "bite" of the brakes but then gave way to nothing as you pressed the pedal further, and then as you pressed the pedal more, normal pressure built and applied the brakes. Today, after the first pump or two, that was the case again but that issue now seems mostly gone. Wondering if my master cylinder has always been wonky.

Can you hear that engage as it builds up air pressure ?

Not sure how I could hear the vacuum booster do anything?

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I looped a clear tube up and over each bleeder, to trap air so that I could do it without a helper. Here's an example picture, of a rear drum, with clothes hanger holding up the loop. So long as there's no air in the "down" slope (leading to bleeder screw) I shouldn't have any air, because each time I released the brake pedal, it could only suck fluid back in this way.



Maybe I'll trying bleeding it all again tomorrow. Trouble is, I don't know what I'd do differently!

 

while that method could possibly bleed an air bubble from a caliper...
It is not a proper method of bleeding brakes.

If yer gonna do it manually it takes two people.
One compressing the pedal fully
and another cracking open the caliper nipple to bleed.

You air bubble is not likely trapped in a caliper
but futher up the line.

Proper bleeding...

rear pass.
rear drivr
front pass.
front drivr

You will need re-fill the master cylinder
when bleeding fully.
Do not let the MC fluid level go below it's fill line.

It takes a little while to bleed manually.

Optionally you could do a power bleed
and then it could be a one man deal.

Either way... you need to bleed it fully.

A brake pedal that returns to neutral promptly
but is spongy in that it can compress some once the pads seat
has me thinking there is still air trapped inline.

Or... the power booster is not working.
as that will stiffen - harden the pedal pressure
when the engine is running and the brake pedal is depressed.

To know if that is working...
depress the brake pedal before start up...
the brake pedal should attempt to raise slightly
when the booster builds pressure.
Sometimes the foot must have less pressure
for the pedal to react to the booster build up.

Tho' I'd first start with bleeding the hydraulic line properly and thoroughly.

 

Thanks for all that. Don't need to also bleed the brake proportion valve?

I'm not clear on why my method wouldn't work to remove air from anywhere in the lines, since air pushed out couldn't get back in.

I'll check the booster in the morning, too.

 

yes you'll likely need do the proportioning valve as well.

because your method will not see a high volume of fluid passed thru the line
as the brake is compressed/held fully when a caliper's nipple is cracked open to bleed.

https://youtu.be/f57r4lqxTC4

You'll want to push thru all the fluid thru, coming from the master cyl.
so it takes more then a few attempts.

You can re-use the clean fluid by draining it
in an appropriate container...
then transferring it back to it's original container to rest...
until the air bubbles dissipate.

I generally go thru two small brake fluid bottles
when doing a full flush.
But once the dirty fluid is flushed thru...
I save the clean fluid for re-use (later)

Some folk say a gravity dump can work too...
but in reality... there are only two sure fire methods.

1. Manual / foot pumping the brake pedal, full compression,
crack open the appropriate caliper nipple to bleed, close.
repeat

2. Or use a power bleeder...
and your life just became simpler
and likely will become a one man show.

Your call...
your problem.

I don't have one !? Lol

 

Yes, I also did the proportioning valve, after doing both rears.

I pushed a 32-oz. container of new fluid through the system while doing all of this. If I understand you, having the bleeder closed when depressing the pedal pushes air out better due to the extra compression, so I guess air moves more under compression than fluid. I'm not arguing, just trying to understand.

The first one (passenger rear) got about 25 or more foot pumps outta me before I was satisfied not only that air was gone but that fluid was new (went from dank green color to basically pale yellow/green color, as from the bottle.)

Each successive wheel took fewer foot pumps, as expected.

 

yep... so did the brake pedal become harder to compress by foot ?

Does the brake peddle still compress or feel spongy during foot compression
...when the engine/brake booster are running ?

 

I think the wood block prevented you from getting all the air out of the MC. You said the MC ran dry, that's where the air bubble is, with the wood block preventing the pedal travel required. Bleed the brakes and allow full pedal travel, MC's are engineered to do that.
You maybe thinking of the wheel cylinders for the rear drums, if you don't set the pins in the shoes right, the WC pistons pop out and make a huge mess.

 

TacoDell, It did become harder as I progressed in the sequence. With engine running and truck parked, the pedal will sponge down if I press hard. This doesn't happen while driving, however. It's not as if I come to a stop sign and then the pedal moves away from me.

Aught2TaCO,

I wasn't thinking of the drums' wheel cylinders, I was just reiterating what I'd read here (I thought) about the master cylinder. So I guess I'm more confused now, but if you say it's OK to push the pedal all the way to the floor, then I'll consider it. I figured that with the wood block there, I'd have to do more pumps, which didn't bother me. Thanks.

 

Yup, perfectly safe, just don't try to push the pedal through the floor. And sorry for confusing you, but I've never read on here or anywhere else, to place a wood block under the pedal. If you have links, please post. Maybe I'm missing something.

 

OK, I'll revisit this in the morning and see if I can find a link.

 

Occasionally a brake booster needs a slightly higher rpm, then say at idle,
to build up enough and hold pressure.

If yer driving the vehicle...
that higher rpm may increase boost pressure better.
And maintain that pressure or at least last a little longer.

When driving...
Occasionally my pedal might compress slightly more on it's initial stab...
and the brakes do not net their full clamping pressure.
But if I hit the brake pedal again the booster stiffens the pedal pressure.

I thought that possibly my issue was attributed to the fact
that my brake booster is now 16 years old
and is possibly not holding its pressure as well as it once did.
Meaning the pressure seal is likely getting tired.
and possibly needs replacement... but I'm not fixing it until it fails completely.

Most the time my brakes feel solid and don't require
the additional stab to build up the specified brake pressure.

My particular braking issue seems intermittent for the most part...
so it may not be related to what you are feeling.

I know for 100% that I have evacuated all air in my hydrl. line, caliper and the MC itself.
So I might hafta' think that my fault can be blamed in having an old, tired booster.

Yes, brake booster seals wear out and don't live forever.

Test that your brake booster is working...
by holding the brake pedal down at start up.

Initially the pedal will go down before engine start up...
So if the booster is working properly...
the pedal should rise back up slightly and firm up during start up.

 

Brake booster

"The booster will never cause a low or soft brake pedal, check for air or other hydraulic problems. A defective booster will only cause the pedal to become hard or not return."

My pedal returns and doesn't have an always-hard issue. I did 3 typical tests to confirm both the booster and the check valve, and it passed them all: http://www.newtonnet.co.uk/coupe/service/webtech/iindex.asp@id=12476

Also read this stuff below; might apply to you. A restriction due to carbon build up should be an easy thing to check.

"Check the vacuum source to the booster. Not only does the booster need at least 15 in. Hg, it also needs vacuum volume. The fitting normally on the intake manifold or carburetor will get clogged with carbon build up and restrict the vacuum volume."

Needing multiple pumps of the pedal apparently describes a condition of weak vacuum. Really poor vacuum = no boost essentially, and an always-hard pedal (that also might not return.)

Kinda fascinating when you think about it: weak vacuum causes multiple pumps until firm, but "no" vacuum causes always firm. No wonder this shit's confusing.

"TIP: An easy test is to park a vehicle with a good brake pedal next to your car and run the vacuum to booster in question. If pedal is OK now, vacuum source is insufficient."

 

This isn't the source I'd initially read it from, but does mention it in the context of "uncovering" previously unused area within the master cylinder, which cases dormant corrosion to break free and screw things up. http://www.agcoauto.com/content/news/p2_articleid/266 I believe the issue I'd read earlier is that when the pedal is pressed to far to the floor, seals can get over-extended (break). Makes for a great rumor if nothing else.

Scary site, also mentions all sorts of things that can go wrong. My favorite one is where they describe weak drum brake seals that let air in upon release but are strong enough not to leak when the pedal is pressed, causing air to never leave even when bled, and no leaks. That type of thing falls into the category of just replacing all sorts of shit even though it tests good. That's a level of insanity I'm not ready for here.

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Drove it again this morning and included some light highway driving. I'm standing pat for now. The pedal pressure, while lighter than it was with the old brakes, is incredibly linear now, with direct action of the pedal corresponding to direct braking action. Credit the SS lines for that?

While stopped, I can press harder and get it to compress. Is that air? Maybe. Could I bleed it some more? Maybe. Does the pedal work a lot better than it did previously, while driving? Absolutely.

 

...I'm not confused

but I'll thank you for your tips
as they may be useful to others.

 

Thanks for the link! Well... I don't know what to say now other than I would still take a chance and let the pedal travel to the floor during bleeding. Whatever you do, please keep us posted here, good luck!