Well the caps should be sealed, so if its leaking, something is not sealed. When you press on the brake pedal, fluid squirts up so it needs to be a good seal.
I just re read your initial post. Did you bleed all four brakes? How did the front ones do? That slow sinking pedal could point to a partially collapsed rubber hose, or a slightly sticking caliper. On some cars, you can also get an air pocket above the front bleeder screw-sometimes have to remove the carrier and tilt it so the bleeder is on the top.
Just trying to throw you a few more ideas.
I am beginning to think that the leaking filler caps on the MC rez and the sinking brake pedal might be two different and unrelated problems. The MC and rear wheel cylinders are new. The lines have been bled. I have excellent braking. The pedal is solid upon braking and then it slowly starts to sink. I discovered no leaks other than from the filler caps and I donāt see how that could be related to the sinking brake pedal, nor do I see how air in the lines, if any, could cause a brake pedal to sink slowly under light pressure. The booster has no vacuum leaks, as I described earlier in this thread. The MC is not leaking from the rear.
I am coming around to the possibility that the new MC could be leaking internally. This is because I cannot think of any other reason for this to be happening.
Thanks, Oldcars, yes I bled all four brakes, RR, LR, RF, LF, for the MC installation. The next day after rear wheel cylinders installation I bled RR and LR, did not bleed RF and LF as I did not open those lines since bleeding them the day before. I replaced the rear wheel cylinders after replacing the MC did not solve the sinking pedal problem, hence the need to bleed those lines twice.
On the first bleed, after replacing the MC, the LF caliper would not bleed when opening the bleeder screw and applying about 20 to 30 pounds of vacuum. I had to pump the brake pedal to get the flow started After the flow started I bled until no bubbles and the brake fluid came out clear.
As I mentioned in a previous reply here, the brakes are solid when applied and I have excellent stopping power. It is only after stopping that the pedal slowly sinks under light pressure.
Just remembered running across this yesterday:
"Does your 4Runner have rear drum brakes instead of discs? If so, worn or out of adjustment rear brake shoes can cause a low pedal and even mimic a failing master cylinder."
Low brake pedal after new pads and bleeding?
I just installed the brake shoes yesterday. Cleaned and lubed the threaded self-adjusting mechanisms, and adjusted to just allow the drums to be installed. I figured that the self-adjust mechanism would fine tune it. I didnāt turn the drums and the shoes need to wear in.
After pedal sinks, one or two pumps brings it right back. And as I mentioned in another reply, the pedal never loses braking power. It sinks just so low and no lower.
Adjusting the rear brakes may be all thatās needed.
Thanks, RK. I will give it a shot and report back
The lids on those MC rezās have to have a little hole in them to the outside air for it to work. Otherwise a vacuum or pressure area might form above the brake fluid and prevent the brakes from working. So it is possible during forceful braking for brake fluid to splash & squirt out that air hole in the lid. On my Corolla thereās a little black plastic thing that floats on top of the fluid to help prevent this from happening. On my Ford truck thereās a rubber bladder liner above the fluid surface that prevents fluid from squirting out the little hole in the top of the metal top on the rez. Concur w/Rod_Knox that checking the adjustment on the rear drums is the next step. I sometimes have that problem w/my drums-all-'round truck, where w/new shoes, the shoes have to be manually re-adjusted after driving it a while .
Re: How would a booster damage the seal of a MC? Unlikely to happen, but it theoretically could I think. The booster has a vacuum moving around inside, and a vacuum creates a force if adjacent to a non-vacuum area, like the fluid in the MC. The vacuum in a booster must at times be adjacent to the rear seal of the MC, b/c one unusual failure mode is that the booster sucks brake fluid from the MC into the intake manifold. Causing black smoke out the exhaust and loss of brake fluid.
I adjusted the rear brakes until there was a very slight drag . The brake pedal engages the brakes at a higher position, but the sinking pedal persists. One puzzling thing is that the sinking is not consistent. Sometimes it does not occur at all, and when it does, which is most of the time, the onset of sinking after the brakes fully engage varies, sometimes right away and sometimes after several seconds pass. The rate at which the pedal sinks to the point where it stops sinking and holds, also varies, faster and slower.
For the time being I am going to live with it to see if I can determine what exactly is causing it. I donāt want to throw a new booster at it, because the booster is doing itās job with no vacuum leak. The pushrod is wiggly, but not overly so, maybe by design, I donāt know, and the MC is not leaking out of the rear. The wheel cylinders are new, and the lines are well bled. The brakes are not spongy, the pedal is nice and hard on braking and it brakes fine. At some point I might throw another new MC at it. I may have gotten one that leaks internally.
Thanks to everyone for all the help and feedback. I really appreciate it.
That thought is not fair to the people who have to share the road with you. You have a problem with your brakes and you need to have a real brake shop solve it before you injure yourself or someone.
Weāve followed this guyās discussions over the years, and Jack strikes me as a guy who is persistent and eventually solves his truckās problems on his own
Check the condition of all the rubber brake hoses (one at each front wheel, one at the rear axle). Have someone press on the brake pedal while you a looking at each hose. Any bulging or other signs of deterioration? I would wear safety goggles while doing this, in case a hose decides to pop while you are doing this.
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Given this OP can eventually solve problems we are talking about brakes here. I still say brakes that donāt work properly donāt solve their selves and never fail at a good time.
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Just came back from a trip to the recycle center in which I had ample time at long red lights to sit with my foot on the brake pedal. There was no sinkage at any time. I did nothing between yesterday when the pedal was sinking and today when it wasnāt. Could it be that the seals expand from contact with brake fluid?
I will certainly check the rubber hoses and will probably replace them no matter what condition I find them in.
Not likely.
How high is the pedal?
From memoryā¦ very high. I could go out there with a ruler and measure, but itās dark. maybe tomorrow.
To VOLVO_V70 who commented, āThat thought is not fair to the people who have to share the road with youā
I bought this truck new and in all these years, no one has serviced the brakes on it but me. They have never failed. Drive with confidence, and please realize that I have my own safety in mind, as well as those who share the road with me. If I thought that my brakes posed a danger to anyone, I would sideline the truck without need of encouragement.
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Iāve had spongy-feeling brakes recover on their own to a firm pedal like that. I think the air bubbles can find their way out sometimes on their own, with enough time. But not with brakes that gradually drop to the floor as you press on the pedal . Thatās always been caused by a defective mc in my own diyāer experience. OP seems to be on the right track.
The mechanics of self servo drum brakes makes it necessary to raise the wheels and run the adjuster star until the shoes begin to drag on the drum, then pound repeatedly on the brake pedal to shift the shoes to their most centered location and then running the shoes out further if they no longer drag. When the wheel can be turned by hand against a minor drag after several poundings on the pedal the brakes are well adjusted.
I had the same experience. Once the pedal was āgoodā a few weeks after I installed a new master cylinder it stayed that way for years (until traded). I concluded the seals āwore inā and started sealing like they were supposed to.