Bad master cylinder?

Well, as luck would have it, a simple brake job takes a turn for the worse. So in doing my pad and rotor job, I had to open the bleeder on one of the calipers to relieve the pressure behind a stubborn caliper piston. Bled the system, and all was well. Well today, I noticed that when I'm stopped and if I mash on the pedal, the pedal skips, or has a popping feeling. Like it hits a wall, then keeps going. Almost like its hung up on something. So I run it to a shop that Ive been to before, pretty reputable, for a system flush and bleed and they notice the problem and bleeding the sytem didnt help it. They seem to think something is causing the master cylinder to bind up. The truck did sit for awhile before I got it, so they think it could be rust or some other foreign object causing the cylinder to bind up. The mechanic said to drive it and see if it gets better over time. The truck hasnt had any issue stopping yet, but driving around with an iffy brake pedal doesn't give me the warm and fuzzies. So, my question here is, does this sound like a master cylinder issue? Toyota quoted me $585 for the replacement, and I may bring it to them just to see what they think before commiting to replacing it.

 

I would bleed it first, look under the dash where the brake pushrod is to see if its leaking

 

I think you are on the right track with the rust/corrosion. The brake master cylinder is composed of 2 separate spool valves and what you are probably feeling is one valve not returning fully due to corrosion build up or voids in the cylinder wall. If that is the case the master cylinder needs to be replaced. If you can bleed brakes you can do the job... is she has ABS it "might" be more difficult.

 

Didnt even think to look under the dash. Would this be simply tightening up a connection? Ive had it bled/flushed at a shop. I will look at it tonite.

 

Every time I bleed brakes, it seems I can never get it quite right. Im just confused as to why this showed up after a simple pad and rotor swap. Pedal will go halfway, then it hits a wall, it pops and I can hear some air escaping from under the dash as I push it the rest of the way. It never really goes to the floor.

 

@Empty_Lord this sound familiar at all?

 

Sounds like a seal could of gone bad in the master.

 

Unfortunately if you see any leaking at the pushrod there's nothing you can tighten to fix it. Seal(s) are bad. The adjustment at the pushrod is on to adjust the brake pedal.

I've never encountered the issue you described. Generally when a master cylinder goes bad (leaky seals) you loose pedal pressure (pedal slowly sinks to the floor as you are applying pressure to it) so it does sound like it could be a possible corrosion issue if your fluid has been green for a long time (indicates moisture in the fluid).

I'm still on my original master cylinder & brake booster. 17yrs old & 215K mi. I've never had a master cylinder on my prev cars last this long. But I have a feeling mine may be on it's way out soon.

 

Fluid is like dark beer piss color.

 

Different brands & types of brake fluid will turn different colors. My old Integra turned that color. Most Toyotas that I see with stock fluid turns green. Either way, if it has been like that for a long time the moisture in the fluid could have possinly cause internal corrosion as you suspected. It has never happened to me though. I barely flushed out my brake fluid this year, I was loooong over due. I think I'm going to flush it out one more time to see of my issue goes away.

 

If the systems never been opened before and not bleed and the fluid looked bad. Chances are there was corrosion past the normal travel of the brake pedal. When bleeding it and putting it to the floor it’s possile it tore the seals.

 

So the master is looking like it. Not a booster issue, correct?

 

Booster is for power assist so it usually cause stiff pedal pressure or engine stalls when they go bad.

 

So Im thinking of taking it to Toyota for their opinion (They wont charge for a diagnostic). If they say its the master, I may just pick up a mityvac bleeder kit off Amazon and do it myself. Any experience with aftermarket cylinders? I wont go reman, but this is more what I'm thinking of: https://www.napaonline.com/en/p/NMAM3519
The truck does have ABS, would this change anything?

 

I’ve had hit or miss with aftermarket masters. The oem is so damn expensive though. I’d rather go aftermarket than bew

 

Olathe Toyota is showing $250 for theirs. https://parts.olathetoyota.com/oem-...jb21hJnk9MjAwNCZ0PWJhc2UmZT0yLTdsLWw0LWdhcw==
The Napa one I can get for around $160 because of the October 20% off three or more things deal. I'm split right now, the Napa one will allow me to get the MityVac bleeder thing and hopefully keep this project under $270.

 

Having ABS increases the potental difficulty in bleeding the system depending on if you let the hyraulic lines bleed dry. Some people have zero issues while others want to junk the truck after many attempts to bleed the system. As an additional test with the motor off, press the brake pedal until the vacuum reserve is bleed off the vacuum booster. Slightly hold your foot on the brake pedal and start her. If the pedal drops slightly the booster passes the first test. With your foot still on the brake pedal shut her off. If the pedal rises as soon as the motor it shut off you are going to either need a vacuum booster check valve or a vacuum booster. The master cylinder, booster and ABS are a system, they all need to be be working properly for the correct brake "feel".

When you are ready to replace the brake master cylinder bench bleed the new master cylinder and have her ready to install, if you have to transfer the resevoir pop it off the original, clean it with warm soapy water and dry it inside and out. There is a master cylinder to vacuum booster seal that needs to come out of the booster and reinstalled on the new master cylinder. Some come equipped with the seal, some dont, you have to be aware that it is there. If you try to install the master without taking it out there is a possibility that you could cut the seal and now you have introduced a vacuum leak into the motor. If the new master has the seal already, take the old one off either the master or the booster and save it... it is not avalible seperatly, as far as I know. When you take the old master off look at the seal where the pushrod goes through. If it is wet there is a possibility that there is brake fluid in the bottom of the vacuum resevoir with the potental to rot out the booster diaphgam.

The minimum amount of time that the brake lines sit and drain the better for bleeding and the less chance that air gets trapped in the ABS controller. This has become such an issue that modern cars have a way, via OBDII, to cycle the ABS pump specifically for bleeding the brakes. You can achieve a similar effect by bleeding the brakes to the point where it does stop the truck drive her on a dirt road and lock the brakes a few times to get the ABS pump to actuate, then go back and bleed the brakes again.

after reading all of the things that can go wrong... aint car's fun?

 

Yet another reason why I'm glad I don't have ABS.

 

Honestly if your using the mityvac I’ve never had an issue bleeding brake systems with abs.

 

Seems like you could divide and conquer by threading in plugs into the master cyclinder. See if the problem persists while taking the brake lines, ABS, calipers all out of the equation.