Master Cylinder?

[KEENO]

Hey First Gen'ers!



My W-250 has changed in Brake Pedal feel over the past few weeks. I'm now starting to loose Brake stopping power during hard stops & the Brake Light has begun to come on near the end of the stop.



I guess it could be (2) seperate things: Master Cylinder or Booster?



Any advice? Also: My rear ABS hasn't worked in years. I don't seem to be loosing Brake Fluid anywhere. NO visible leaks and the level is fine.



The ole girl is showing 302,000+ on the OEM Master Cylinder & Booster.



TIA..... Tim "KEENO" Keen

 

[bmoeller]

Is the pedal getting hard to push, or does the pedal feel spongy? If it is getting hard, it would most likely be a vacuum problem, whether it is the booster itself or the pump took a dump and anything in between. May be a check valve on the booster that could be bad.



On my Ford, the booster had a hole in the diaphram. Near impossible to to push the pedal at all.

 

[dan brooks]

When my master cylinder went south, the brake light would come on during hard stops or when trying to stay stopped on a steep hill. It never dropped all the way to the floor, but there was noticable difference in feel from normal. I was loosing a little fluid though, through the master cylinder into the booster, so I swapped out both.



Does the booster hold vacuum for any length of time after the engine is turned off? If not, odds are the booster or check valve is bad. I'll try to dig up the FSM and post the test procedure for the booster.



If you end up replacing the booster, you might as well put a new (not rebuilt) master cylinder on at the same time. I think most parts stores require installing a new/rebuilt MC with the booster to maintain the warranty.



Can't help on the ABS - haven't had to mess with that yet.

 

[dpuckett]

Sounds to me like a master cylinder. To my understanding, all the booster does is assist in pushing the pedal down. If you get a master cylinder, get a NEW one; the remans arent worth the cardboard to make the box they come in. I've had trouble with new ones, even. Be sure you get the RWAL valve bled out well. You may have to disassemble it and clean everything out and put it back together. I did that on my 91, and it made a world of difference. AFter that, flush the old fluid out yearly to keep everything cleaned out and prevent corrision.



Daniel

 

[KEENO]

Thanks Guys!

Thanks for all of the tips! I, too, thought it was a Master Cylinder. I swapped it out tonight w/ a Mechanic Buddy's help (He thought MC also) and still have the issue.



To answer a few questions:



The Booster feels fine! It has never gotten harder as the miles pile up. (maybe, a tad softer depending on outside temp)). It holds pressure well... even during morning startups after sitting for long periods of time. We both thought Master Cylinder because if feels decent.



My issue is very similar to Dan Brooks: The light will come on during the last portion of a hard stop. The pedal doesn't travel much further..... It just changes in feel (Brake Pressure).



We did not replace the Booster yet and might hold off a couple of days.



What else? It almost feels like the RWAL is kicking in during the final portion of the stop. The Ironic part is..... RWAL hasn't worked since I've owed the truck (1999/2000).



Do I purchase a new Booster &/or investigate the RWAL?



Thanks... ... . KEENO

 

[KEENO]

One more thing.......

While bleeding the Rear Wheel Cyl(s) w/ the Engine OFF. It was noted that rear shoes would "Relax" as I held the Brake Pedal before the bleeder was OPENED. (The Booster wouldn'r be functioning w/ the engine off... . Correct?)



Kind of like the "pressure" was being Bypassed! RWAL? We didn't take it apart tonight before it got Dark.



KEENO

 

[dan brooks]

Sorry I didn't post up the test procedure for the booster earlier... been fighting a cold and it slipped my mind. Anyway, here it is:



Master cylinder/power booster test

1. Start engine and check booster vacuum hose connections. Hissing noise indicates vacuum leak - correct any leaks before proceding.

2. Stop engine and shift into neutral.

3. Pump brake pedal until all vacuum reserve in booster is depleted.

4. Press and hold brake pedal under light foot pressure.

a. If pedal holds firm, proceed to step 5.

b. If pedal does not hold firm and falls away, master cylinder is faulty (internal leakage)

5. Start engine and note pedal action.

a. If pedal falls away slightly under light foot pressure then holds firm, proceed to step 6.

b. If pedal pressure is high, or no pedal action is discernible, power booster or vacuum check valve is faulty. Install known good check valve and repeat steps 2 through 5.

c. On diesel models, vacuum pump hose or pump compontent may have malfunctioned. Check pump output with vacuum gauge and repair as necessary.

6. Rebuild booster vacuum reserve as follows: Release brake pedal. Increase engine speed to 1500 RPM, close throttle, and immediately turn off engine.

7. Wait a minimum of 90 seconds and try brake action again. Booster should provide two or more vacuum assisted pedal applications. If vacuum assist is not provided, perform booster and check valve vacuum tests. Also check vacuum output on diesel models.

 

[dan brooks]

Power booster check valve test



1. Disconnect vacuum hose from check valve.

2. remove check valve and valve seal from booster.

3. Using hand-operated vacuum pump, apply 15-20 inches vacuum at large end of check valve.

4. Vacuum should hold steady. If gauge on pump indicates any vacuum loss, valve is faulty and must be replaced.



Power booster vacuum test



1. Connect a vacuum gauge to the booster check valve with a short length of hose and a T-fitting.

2. Start and run engine at idle speed for one minute.

3. Clamp hose shut between vacuum source and check valve.

4. Stop engine and observe vacuum gauge.

5. If vacuum drops more than one inch vacuum within 15 seconds, either booster diaphram or check valve is faulty.

 

[dan brooks]

Testing diesel engine vacuum pump output



On models with a CTD engine, a low vacuum condition in the booster will cause the brake warning light to illuminate.



1. Check pump vacuum and booster hoses and connections. Make sure hoses are in good condition and securely attached. Run engine and check for vacuum leaks. Replace leaking hoses before proceeding.



2. Disconnect vacuum hose at booster and connect vacuum gauge to hose end.



3. Run engine at curb idle speed and note vacuum reading. Then run engine at 1/2 to 3/4 throttle and note vacuum reading again.



4. Vacuum should range from 8. 5 to 25 inches vacuum at various throttle openings. Vacuum should hold steady and not drop below 8. 5 inches.



5. If vacuum output is okay, check booster and check valve as described. If vacuum is low, or does not hold steady, vacuum hoses or pump components are faulty.



EDIT: If you need info on "diagnosing low vacuum output" let me know - my fingers are getting tired.

 

[Moose Truck]

On my '89 the pedal was spongey twice, Once due to an about to blow brake hose. Frame to axle, The pedal got frim after I replaced the hose. 2, The rear brakes had a bad leaky Slave Cylinder, the pedal firmed up after changing it. Hope this helps, Moose

 

[arrowsmith]

when my booster went bad both brake lites came on. RWAL and brake lites

 

[TKingsbury]

Maybe by-pass the RWAL valve and retest. I don't know about anyone else, but I really hate the anti-lock on these old trucks. It is so intrusive and it seems to about double your stopping distance when it kicks in (as if it wasn't long enough already). My . 02

Travis. .

 

[53 Willys]

Mine had a bad wheel bearing- Just enuff play to knock the pads back in and the peddle would almost hit the floor. With the rotor size on these trucks it does'nt take much

 

[KEENO]

Thanks!

Thanks Dan Brooks!



I appreciate you taking hte time to type out the test procedures! I understand about that tired finger syndrome... I only use one or two at time when I type!



I'll let you know how it all works out!



Keeno

 

[MMiller]

I wonder if you need to adjust the rear brakes up tight. Maybe they are having to travel too far?



I have replaced my master cylinder, twice, once cause it was bad, pedal started getting soft and leaked out the rear, and the second time because the first replacement was junk! At the time of the second replacement, I adjusted the screw between the m/c and the booster out. In fact I adjusted it too far, took the truck for a drive and thought the brakes were draggin. When I loosened the m/c the truck moved slightly proving my thought. Anyways I adjusted it out and now my brake pedal has less free travel then any new truck I have driven. It has been this way for at least two years now, and I have not had to adjust the brakes for at least 10,000 miles either.



Good luck, and good to hear from you again Tim.



Michael

 

[KEENO]

Mind Reader!

Hey MMiller!



You're reading my mind! I checked w/ the folks that service my Brakes/Tires and the rears were done at 248,000 miles / May 2003. Today, she's sitting at 303,000 w/ only one manual rear adjustment made during the last 55,000 miles.



I plan to manually adjust the rears tomorrow after turkey & dressing and see what I get!



Thanks for the Help! Good to be back!



Tim "KEENO" Keen



Happy Thanksgiving to ALL!