Brake problems..........
Thread Starter
|
Postmaster
Joined: Jun 2008
Posts: 2,500
Likes: 2
From: Michigan
Brake problems..........
I just completely redid the rear brakes on my truck after it had been driven on the front brakes for years. It stops alot better now, but when the truck is running the pedal goes all the way down after you have stopped and are sitting there with your foot on the pedal. Im thinking something is up with the master cylinder. What do you fine folks think?
Postmaster
Joined: Apr 2008
Posts: 4,802
Likes: 1
Typical symptom of internally-bypassing master cylinder, what happens is some of the fluid from the high-pressure side leaks past the piston seals and thus you slowly lose fluid and pressure, hence the sinking pedal. A new master cylinder should cure that, however when you remove the one you got now take a look at the brake booster and see if its paint under the master cylinder is all bubbled out or peeling - it's not uncommon for master cylinders to also leak fluid past their rear seals and that fluid to end up in the brake booster, once it's inside the booster it will eventually damage the diaphragms - that's actually why if you're buying a new boosters some manufacturers have the requirement that a new master cylinder be installed at the same time, or they will not honor warranty should something happen to the new booster.
Thread Starter
|
Postmaster
Joined: Jun 2008
Posts: 2,500
Likes: 2
From: Michigan
Whats all them letters stand for? Im thinking RABS is rear antilock brake system, but no idea on the HCU???
Trending Topics
Senior User
Joined: Apr 2006
Posts: 478
Likes: 0
From: houma, louisiana
how well did you adjust your rear brakes. push the emergency brake petal down and see how far it goes down
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
Post Fiend
Joined: Jan 2005
Posts: 8,402
Likes: 10
There are two solenoid valves in it, one can open and allow brake fluid to flow into a chamber against a spring loaded piston. This should only happen under heavy braking and when both rear wheels are locked up.
However if the valve is faulty it can allow fluid into the chamber under normal braking and so the pedal slowly sinks. When the pedal is released the piston pushes fluid back to the resevoir so it all happens again the next time you press the pedal.
It is easy to find out if that is the problem. If you look at the valve module you will see a large hex bung. Unscrew this bung (against spring pressure all the way) and remove the spring. No fluid will come out. Replace the bung, pump the pedal a few times to fill the chamber and then see if that has made any difference. If no difference then replace the spring and look elsewhere.
Postmaster
Joined: Apr 2008
Posts: 4,802
Likes: 1
That'd be your booster, they all do it, as tis part of their operation cycle - upon pedal release the vacuum that just got used to pull on the diaphragms gets released, the air flowing in makes that sound you hear.
Postmaster
Joined: Jun 2007
Posts: 2,621
Likes: 1
From: Sweetwater Texas
You can buy the entire assembly (master cylinder booster combo) from O'reilly's for 80 bucks. You HAVE to bench bleed the master cylinder tho, and be sure to bleed it fully or else you will be fighting air for weeks. It comes with instructions on how to do it and its not hard at all it just takes 15 minutes of time.