E350 Master Cyl Help!
Thread Starter
|
Trailering
Joined: Sep 2024
Posts: 17
Likes: 0
E350 Master Cyl Help!
- I have a 1993 E350 7.3 IDI Ambo. I have wrenched a long time but never replaced a M/C.
- I am driving this 200 miles a week.
- Had ABS and Brake Warning for a few weeks - redid rears with cyls and tubes, shoes.
- Bled all and RABS, brakes still sinking at standstill, not great, but working
- Went to replace M/C with new M/C as old fluid was dark and figured seals were shot.
- Noticed reservoir was so empty, shockingly so but I cannot figure out where it went. Booster?
- Not on ground and am going to double the system for leaks. To that end I ask.
1993 E350 7.3 ambo m/c
Hotshot
Joined: Jul 2020
Posts: 10,534
Likes: 2,664
From: Near Salt Lake City
The brake fluid leaked out the back of the master cylinder and into the brake booster
Get the master cylinder replaced and double check the low or sinking pedal
You may need a RABS valve for the sinking pedal
Get the master cylinder replaced and double check the low or sinking pedal
You may need a RABS valve for the sinking pedal
Thread Starter
|
Trailering
Joined: Sep 2024
Posts: 17
Likes: 0
Thanks
Thank you for your reply. Just replaced the MC. My notes for future readers.
1993 Ford E350 cutaway rear 4:11 w/ 3.5 inch wide shoes. @44,000 miles. Five tons GVW. Driven weekly in all traffic scenarios except towing.
Symptom : Problem started with an error light and stuck parking brake after applying the parking brake thinking it was the reason for the brake warning light. 2) Brakes worked then rear passenger side smoked at highway speed.
Inspection: Broken rear brake shoe return springs on both sides from corrosion. Both rear brake cyl weeping. Corroded lines. All of these items replaced rear only. Bleed all wheels. Brake need multiple pumps to stop.
Rework: Tighten loose nut and bleed all wheels and RABS. Sinking pedal but stoppable without a pump. Vac leak to brake booster ruled out due to recent vac pump replacement and fully functional HVAC controls. and a vac leak gives a hard brake pedal.
Rework: MC replaced, unknown residue observed on booster exterior and MC end of replaced part. Fill with brake fluid.
Test: Pedal engaged higher and firmer. Brake performance improved but still sinking to 1/4 from full travel. Brake and ABS lights illuminated. Needs full bleed. - rear pass, rear driver, RABs, Front Pass, front driver.
1993 Ford E350 cutaway rear 4:11 w/ 3.5 inch wide shoes. @44,000 miles. Five tons GVW. Driven weekly in all traffic scenarios except towing.
Symptom : Problem started with an error light and stuck parking brake after applying the parking brake thinking it was the reason for the brake warning light. 2) Brakes worked then rear passenger side smoked at highway speed.
Inspection: Broken rear brake shoe return springs on both sides from corrosion. Both rear brake cyl weeping. Corroded lines. All of these items replaced rear only. Bleed all wheels. Brake need multiple pumps to stop.
Rework: Tighten loose nut and bleed all wheels and RABS. Sinking pedal but stoppable without a pump. Vac leak to brake booster ruled out due to recent vac pump replacement and fully functional HVAC controls. and a vac leak gives a hard brake pedal.
Rework: MC replaced, unknown residue observed on booster exterior and MC end of replaced part. Fill with brake fluid.
Test: Pedal engaged higher and firmer. Brake performance improved but still sinking to 1/4 from full travel. Brake and ABS lights illuminated. Needs full bleed. - rear pass, rear driver, RABs, Front Pass, front driver.
Hotshot
Joined: Jul 2020
Posts: 10,534
Likes: 2,664
From: Near Salt Lake City
Still sinking pedal means your RABS valve is bypassing
Test is to block off the rear port of the master cylinder and re test drive it
Then, replace the RABS valve when the pedal no longer sinks with the plug installed
Test is to block off the rear port of the master cylinder and re test drive it
Then, replace the RABS valve when the pedal no longer sinks with the plug installed
Cross-Country
Joined: May 2015
Posts: 71
Likes: 11
I’m curious that you ruled out the brake booster.
I just fixed a vacuum leak at the booster. HVAC worked but engine wasn’t running great and mpg was falling off. Check valve on booster was okay but the booster itself was gulping air. When I pulled it, a pint of dot3 dumped on the floor…so I ended up with a new mc too.
I believe the dot fluid eats the diaphragm in the booster. Seems like if you know you lost fluid thru the seal, the booster is automatically suspect, no?
Asking to check my understanding, not to be a jerk.
I just fixed a vacuum leak at the booster. HVAC worked but engine wasn’t running great and mpg was falling off. Check valve on booster was okay but the booster itself was gulping air. When I pulled it, a pint of dot3 dumped on the floor…so I ended up with a new mc too.
I believe the dot fluid eats the diaphragm in the booster. Seems like if you know you lost fluid thru the seal, the booster is automatically suspect, no?
Asking to check my understanding, not to be a jerk.
Hotshot
Joined: Jul 2020
Posts: 10,534
Likes: 2,664
From: Near Salt Lake City
I’m curious that you ruled out the brake booster.
I just fixed a vacuum leak at the booster. HVAC worked but engine wasn’t running great and mpg was falling off. Check valve on booster was okay but the booster itself was gulping air. When I pulled it, a pint of dot3 dumped on the floor…so I ended up with a new mc too.
I believe the dot fluid eats the diaphragm in the booster. Seems like if you know you lost fluid thru the seal, the booster is automatically suspect, no?
Asking to check my understanding, not to be a jerk.
I just fixed a vacuum leak at the booster. HVAC worked but engine wasn’t running great and mpg was falling off. Check valve on booster was okay but the booster itself was gulping air. When I pulled it, a pint of dot3 dumped on the floor…so I ended up with a new mc too.
I believe the dot fluid eats the diaphragm in the booster. Seems like if you know you lost fluid thru the seal, the booster is automatically suspect, no?
Asking to check my understanding, not to be a jerk.
Brake booster failure gives you a hard pedal in most cases, or a pedal that gets "sucked" to the floor in case the spring inside the master cylinder breaks
Anytime you have no brakes and the pedal sinks to the floor, you are generally looking at fluid bypassing
Either bypassing in the master cylinder or the RABS valve in his case
For his situation, the first thing you check for is a brake fluid leak
If you find no leak and the pedal goes down there must be fluid bypassing somewhere
Cross-Country
Joined: May 2015
Posts: 71
Likes: 11
Cool, thanks for the explanation about the booster.
If HenryIV has dot3 inside the booster though, does it make sense to proactively replace it while the MC is off?
Or was my leaking mc + ruptured booster just a coincidence and not due to fluid eating the diaphragm, as I assumed?
Boosters aren’t cheap but, in my van at least, the MC has to come out in order to remove it, so the time savings down the road is potentially worth it?
If HenryIV has dot3 inside the booster though, does it make sense to proactively replace it while the MC is off?
Or was my leaking mc + ruptured booster just a coincidence and not due to fluid eating the diaphragm, as I assumed?
Boosters aren’t cheap but, in my van at least, the MC has to come out in order to remove it, so the time savings down the road is potentially worth it?
Trending Topics
Hotshot
Joined: Jul 2020
Posts: 10,534
Likes: 2,664
From: Near Salt Lake City
Missing brake fluid is generally always inside the brake booster
I feel no need to replace a booster simply because it's full of brake fluid
Happens all the time and 20 years later they are still okay in most cases
I'd leave the booster alone until you have a hard pedal one of these years
I feel no need to replace a booster simply because it's full of brake fluid
Happens all the time and 20 years later they are still okay in most cases
I'd leave the booster alone until you have a hard pedal one of these years
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
Hotshot
Joined: Jul 2020
Posts: 10,534
Likes: 2,664
From: Near Salt Lake City
On failing boosters
Climate makes a BIG difference
The recalls for boosters all start in TX and FL or the humid southern states and CA
In the desert southwest and places like UT and ID have very little trouble with boosters rusting out
Been inspecting and replacing boosters under warranty and recalls for years and years,
I agree, they fail
I just don't replace them for brake fluid intrusion is all
Climate makes a BIG difference
The recalls for boosters all start in TX and FL or the humid southern states and CA
In the desert southwest and places like UT and ID have very little trouble with boosters rusting out
Been inspecting and replacing boosters under warranty and recalls for years and years,
I agree, they fail
I just don't replace them for brake fluid intrusion is all
Thread Starter
|
Trailering
Joined: Sep 2024
Posts: 17
Likes: 0
I suspect that there is brake fluid in my booster, as it had to go somewhere and I did not see any leaks at the wheels, I do not suspect the diaphragm is gone (yet). As for leaks in the booster, I replaced the VacPump recently and the HVAC controls regained function, that is my differential diagnosis that I do not have a vac leak. As for a fluid change, I replaced the rear cylinders and lines so a full brake fluid replace has already happened. I'm on my 3rd quart of brake fluid on the journey. Tomorrow will be the first time for me to bleed correctly after the M/C replace. I am driving this truck every week for work so I work all week, repair all weekend, work all week, and it has been going this way for weeks. I am running out of things broken.
Thread
Thread Starter
Forum
Replies
Last Post
bulletpruf
1987 - 1996 F150 & Larger F-Series Trucks
41
Jan 27, 2025 07:54 AM
The Prospector
1994.5 - 1997 7.3L Power Stroke Diesel
9
May 11, 2017 07:14 AM