Greetings all! I have a 1991 F-350, 7.3 diesel. Lately the "brake" light and the "anti-lock" light in the dash cluster keep coming on and staying on for long periods of time. Also, it feels like the brake pedal loses some pressure when you hold it down after pumping it. The front and rear pads/shoes look ok. The master cylinder is brand new. The vehicle doesn't seem to be losing brake fluid anywhere that I can see. Any idea's on what could be wrong and how I can fix this? Thanks so much.

 

A diesel does not have vacuum, that is why they have a vacuum pump as an accessory drive which creates vacuum for the brake booster and heater controls.

 

If the brake fluid 'low level' sensor isn’t plugged in, or more commonly have bad/corroded connectors, it can turn on both the ABS and Brake Warning light.. It's located at the base of the master cylinder reservoir.. Check the connectors on both sides ‘carefully’. The spongy/creeping brake pedal has been an issue somewhat frequently, for gassers ‘and’ diesel alike for many MY’s and for various reasons.. So, if you've thoroughly bleed the system after the recent M/C change, and don't have any problem with the RABS valve, or any other brake hardware/adjustment problems this may be an option.. One of the problems is that if you repeatedly pump a diesel pedal you tend to quickly exhaust the pumped/available vacuum during low RPM .. Remember, Ford considers this brake operation ‘Normal’ to a degree. Some of the other pedal concerns for low/soft pedal or volume issues may be noted, by clamping/pinching off the front brake hoses and the pedal height may rise from believed hose/caliper flex, and/or possible lack of volume, which the larger bore cylinder helps to improve (clamping off the hoses usually helps diagnose the RABS as well by isolating front/back hydraulic concerns), and some are remedied with a new RABS, or may at least require a closer look.. If the pedal still remains the same for creep, or too soft, then the vacuum pump may be failing, and if not, you may benefit from this booster improvement.

Here's the TSB in regards to a fix and your issue, “if all other brake components test OK”.. This Ford Booster is backwards compatible from dates shown in the TSB to the cover 1991 MY.. Maybe earlier.. Price is about 150.00..

1997 and earlier
diesel trucks equipped with vacuum power brakes--Low/creeping
pedal:

Many owners have complained of a low or creeping brake
pedal on their trucks, often mistaken for a bad master
cylinder or Rear ABS valve. If the condition occurs after the
vehicle has come to a stop, and no other brake concerns are
found, (worn or out-of-adjusted brakes, cocked pads, or
hydraulic problems) the condition is normal and the result of
the vacuum pump replenishing the vacuum inside the power
booster. Ford has developed a Zero-Loss Travel brake booster
to correct this condition if the customer finds it
unacceptable.

This part is only for use on diesel-equipped
trucks--P/N F5TZ-2005-CA--and is not a service replacement; it
must be ordered by the part number, not vehicle application.
This condition is similar to the low brake pedal concern on
all 1995 F-series that required a larger-diameter master
cylinder: Trucks without cruise control: F6TZ-2140-AB F-250
Gas 4X2; F-350 DRW, SRW Gas 4X2, SRW 7.3 4X2 168' WB
F6TZ-2140-EB F-250 4X4, 7.3 4X2; F-350 SRW 4X4, SRW 7.3 4X2
133' WBTrucks with cruise control: F6TZ-2140-BA F-250 Gas 4X2;
F-350 DRW, SRW Gas 4X2, SRW 7.3 4X2 168' WB F6TZ-2140-FB F-250
4X4, 7.3 4X2; F-350 SRW 4X4, SRW 7.3 4X2 133' WB

These part numbers are from TSB's 96-25-19 and 98-5A-34, and
are not normal service-stock replacement parts.