New Brake Master Cylinder, Brake Booster, but still have squishy brakes.

Hello,

Looking for some help here as I'm really scratching my head...I have 1998 2.7L Prerunner Xtracab. I recently had my original brake booster go bad. No pressure whatsoever on the pedal no matter how hard I pressed.

I replaced it with this SKB model from Rockauto: https://www.rockauto.com/en/moreinfo.php?pk=11137597&cc=1317623&pt=1884&jsn=1

I also purchased a new gasket that goes between the master cylinder and brake booster from the Toyota dealership near me.

Initially, I thought that would do the trick, and it did help, but the brake pedas has a decent amount of travel before the brakes feel engaged. There's also an audible "hiss" noise while the pedal depresses before the brakes engage, even on the new booster. Here's a video demonstrating: https://youtube.com/shorts/d3Ikkb57p1k?feature=share

So I decided from here to get a new Master Cylinder, I bought this model: https://www.autozone.com/brakes-and...last-brake-master-cylinder-nm55097/493950_0_0

Literally zero difference with the new Master Cylinder. I've bled the brakes thoroughly using the water bottle & hose-line method at each wheel starting with the furthest back and working my way up to the driver's side.

No idea what to do next...Do I have a shit brake booster?

 

You need to bleed the rear proportioning valve, that should help, but it just sounds like you have air in the system. A pressure bleeder may help.

 

You should replace the rubber brake lines. They can expand under pressure and cause a soft pedal. Really bleed the brakes including the rear proportioning valve. Adjust your rear drums too. If you have ABS you will need to do an abs bleed as well.

 

When the brake booster goes bad, it gets real hard to push on the brake pedal.

Kinda like when you lose the power steering power/boost, it should get real hard to turn, same theory.

there is a bladder that helps push down the pedal, so the booster had nothing to do with this.

Then next question is, why is the master not pressurizing. My guess is neither of these was the culprit.

So where to look next. Is it possible that both masters were bad, what's the chances of that.

I have seen where tundra wheel cylinders where used, and that extra volume gave extra long travel on the pedal, but it still managed to stop the truck. Any chance you got the wrong wheel cylinders

It becomes guesses after that, maybe you were sucking air back up that bleed line, either thru the line, or around the threads of one of the bleed nipples, any chance the rear shoes are way backed off.

Or you didnt breed aggressive enough to get trapped air out that lspv valve area. Or an air cavity trapped in one of the calipers or wheel cylinders.

Any chance this has ABS?

Running out places to look.

The whole system is just stupid simple, I dont get it.

Two years ago I replaced everything but the booster. The first master was an OEM toyota one I order from Napa, took em a week to get it in.
That one would not pressurize right out of the start gate. So I had to do a hail mary move and go grab a Fenco one at one of the AutoZone's on the other side of town.

I used a whole quart of the Prestone syn max, and that one fired right up.

I love a good mystery, just not when I'm the victim.

 

If you installed the master cylinder right into the truck, it might be an issue. You usually bench bleed the master cylinder before it’s installed. It can be a pain to get the air out of it when it’s installed

 

Yeah I didnt even think about..brain fart...best way to do that....is mount in a vise, and bench bleed it...
cause yes, you have to be very thorough, over do it if anything.

Thankyou for that.

I believe most MC kits come with those MC bleed hoses.

That's the reason YOU NEVER LET THAT MASTER CYLINDER RUN DRY.
if you do, you have to over do it again.

Thankyou again....I really hate failure, mine or anybody elses.

 

CSnipe5
I went thru entire brake system bc of soft pedal

Prior owner RR-d MC and pads and shoes/2 front rotors/2 rear drums a yr ago
I then RR-d all rubber lines
I saw front pads(new) and they were half life….rear shoes still looked brand new……strange….i RR-d front pads again
and bled brakes(even bought one of those vacuum hand pumps)
No change

Then OSilver advised me that I didn’t bleed LSVP and that those things go out all the time

My next project is LSVP delete with $40 proportion valve

I’m thinking that non working LSVP valve was/is issue the entire time
I.E. taco stopping with front brakes only
Proven by zero wear of rear shoes
(Prior owner’s invoice says MC/all pads replaced at same time)

 

Yeah I was anal about those rear drums.

I went and ordered new spring kit, adjusters, cable e-brake paws, Aisin cylinders all oem from Napa.

when I put all that together I made sure to use brake grease where they contact and slide on the dust shield.

I ran $28 bendix shoes 1st year, and the front ones were wearing way too fast.

So I ordered the $70 oem ones next time I had a big order from Mcgeorge.

That was about a year ago, so its time for a check up.


And yeah about twice a year I have to haul scaffolding, so other than that my rear load never changes, so first time that lspv gives me trouble, I got a link saved on parts needed to switch to that manual one...

thats that thread I let you know about.

 

Great point. The original lines can break down and swell under pressure and when they get really bad they can cause issues when bleeding if you use the stomp and bleed method.

 

For those coming back to this I found the solution - bleed after bleed and no changes were made.

Turns out the stem of the brake booster I purchased is a fair bit shorter than the stock one. So I had to adjust the clevis behind the brake pedal basically as far out as it could possibly go and now I have little travel and a firm brake finally.

 

That is so obvious that of course nobody thought of it! Glad you got it sorted.

 

Lol...sometimes it's the most obvious things we miss.
I did delete that Load Sensing Proportioning Valve though, one of the better changes I've made on my 98.

 

They are supposed to come pre adjusted. There is a special tool that measures the depth/length… not expensive.

Was yours clicking on occasion?

https://www.aliexpress.us/item/3256...356316&afSmartRedirect=y&gatewayAdapt=glo2usa

 

holy moly
Thanks for update