hard to soft brake pedal - Diesel Forum

djj454 · 02-28-2005, 01:23 AM

2002 F-250 4x4 psd 50k-- posting again before interrubed this afternoon, first time to hit brakes pretty normal pedal pretty firm, second time very spongy and almost to the floor, third time hitting the brakes too the floor, always first time after about 15 to 20 sec. you will good brake pedal, no lost of fluid, the only thing that I did a few days ago ,changed the brake fluid, it was vary nasty, and I mean vary nasty. Blead the lines and every thing was great, pedal felt great and firm, next morning first 1 to 5 miles spongy butt cleared up. MORNING AFTER that spongy all day and to floor. Got home tonight while truck was sitting still pumped brakes about 15 times, noticed powerstering fluid squirting out of top of cap. Dont understand fine one day and not the next. NEED HELP THANKS for any reply

FMTRVT · 02-28-2005, 03:37 AM

Probably the master cylinder has an internal seal that is damaged. This often happens after a brake fluid change when the master cylinder has been stroked to the max.

The back of the cylinder bore does not normally get used during normal driving, and if the fluid has not been changed routinely, there is oxidation that builds up there. With the seals going over this abrasive oxidation, they can develop a tear or an edge get rolled over.

fordmastertech · 02-28-2005, 07:40 AM

I agree on the master cylinder theory. The best way to change the brake fluid is to use a power flusher or gravity bleed the system by opening the bleeders and letting them drip fowled up by a normal pedal type bleed. Don't push the brake pedal to the floor when bleeding. You may also have some air trapped in the 4 wheel ABS system if you have that on your truck.
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KB · 02-28-2005, 10:28 AM

Boy do I know that feeling. I agree with FMTRVT and FordMastertech. The mastercylinder is probably your problem. I been through this same problem with: a 72 and a 73 Datsan 240Z, a 69 Austin Healy, 79 CJ5 Jeep and I am fighting it right now with a 85 Toyota 4Runner.

Bruce

djj454 · 02-28-2005, 10:34 AM

Thanks for your help and others, yall are great! Back to the brake issue I do have ABS braking. Do you think I may have air still trapped in the system or what, and if i do what would be the best way to bleed the air out. What I dont understand is the powerstering fluid squirting out of the capp, when i sit there and pump the brakes up about 8 to 9 times. Also can I rebuild the mastercyclnder or is it is to go buy a rebuildt one, tring to save a little money if you know what i mean. Going to flush powerstering unit any suggustions. THANKS for all of your help and others

fordmastertech · 02-28-2005, 11:03 AM

If the engine is not running when you pump the brakes or when you turn the steering wheel, fluid will come out of the power steering pump, this is normal. To bleed the ABS you have to use a scan tool that has the ability to do a ABS service bleed. It runs the ABS pump and opens the valves in the HCU to bleed the air out and is followed by another normal pedal bleed.

FMTRVT · 02-28-2005, 12:13 PM

Another trick you can do to dislodge air in the ABS circuits so it can be normally bled out is to take the vehicle on a dirt or gravel surface and lock up the brakes from 25-30 mph. We end up having to do this when we get a prototype vehicle and the NGS or whatever will not recognize the new ABS software.

fordmastertech · 02-28-2005, 12:23 PM

[ QUOTE ]
Another trick you can do to dislodge air in the ABS circuits so it can be normally bled out is to take the vehicle on a dirt or gravel surface and lock up the brakes from 25-30 mph. We end up having to do this when we get a prototype vehicle and the NGS or whatever will not recognize the new ABS software.

[/ QUOTE ]
Good info here, thanks.

djj454 · 02-28-2005, 01:05 PM

Thanks FordMasterTech and FmTrvT for that imformation, do yall think I could possible have air in system or bad mastercyclinder, since on the 2nd day i drove it , pedal went to floor once or twice and then fine the rest of the day. Is it hard to rebuild a mastercyclinder and how much is one from parts house. THANKS GUYS

FMTRVT · 02-28-2005, 01:26 PM

Air in the system is a soft pedal all the time that can have the pedal pumped up with repetitive hits. A bad M/C seal can be on-off with the issue, or continuing loss of pedal or more frequent occurrences of circuit failure.

While to most people it seems like you have lost all of your braking ability, you really still have half of the system working. It's just that it's at the last 1/2" or so of pedal travel. And half is, well, half!

Not sure if you can get rebuild kits anymore due to liability reasons. And if you can, it's a fine line of honing out the M/C bore to not have it oversize and the seals not work correctly (hence the liability).

djj454 · 03-03-2005, 11:49 PM

Went and got a mastercylinder today, Does anybody know the correct or best way to change them out, need to doit tomorrow, any HELP on insulation would be greatly appriciated. THANKS

fordmastertech · 03-04-2005, 12:51 PM

Bleed the new master cylinder on the bench first buy filling it up and letting the fluid drip out. Next use a screw driver and push the master cylinder piston to the bottom of the bore, cover the holes where the lines attach with your fingers and release. Do this several times to get all the air out and then install the new master cylinder in the vehicle. If all goes well you may not need to re bleed the complete system again.
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