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1979 Ford Bronco/F100-F350 Brake & Transmission Vacuum Supply Troubleshooting
Reference · 1978-1979 BroncoAll 48 service procedures
Service-manual page explaining how the power brake booster, automatic transmission vacuum diaphragm and vacuum reserve tank work, with vacuum-gauge trouble-shooting steps (18 in-Hg hold spec).
Part of 1978-1979 Ford Bronco Service Procedures
This page, titled "Brake / Transmission Vacuum Supply," comes from a Ford truck reference manual (uploaded as page 46 of a 1979 Bronco/F100-F350 document set). It explains how the vacuum supply system works and how to troubleshoot it. The system consists of three components: the power brake booster, the automatic transmission vacuum diaphragm, and the vacuum reserve tank. The page describes operation of each: the booster supplies vacuum to a diaphragm to assist braking force; the transmission vacuum diaphragm connects to the intake manifold and modulates hydraulic throttle pressure for smooth shifting; and the reserve tank stores vacuum as a steady source for heater/AC components. The trouble-shooting sections give step-by-step diagnostic flows: for the power brake booster, checking master cylinder fluid, verifying at least 10 inches (Hg) of vacuum at the booster feed and intake manifold, and performing a pedal "fall away" test; for the transmission vacuum diaphragm, checking fluid level, verifying 10 inches of vacuum, and testing whether the diaphragm holds 18 inches (Hg) with a vacuum tester; and for the reserve tank, vacating the tank to 10 inches and checking whether the reading holds, replacing the tank or check valve as indicated. A note refers readers to Ford Transmission Publications for further information.
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Frequently asked questions
- What components make up the brake/transmission vacuum supply system?
- The system contains a power brake booster, an automatic transmission vacuum diaphragm, and a vacuum reserve tank.
- How do I test whether the power brake booster is working?
- With at least 10 inches (Hg) of vacuum supplied to the booster: place the transmission in NEUTRAL, turn off the engine, release the parking brake, pump the brake to exhaust vacuum, then start the engine while applying pressure to the brake pedal. If the pedal tends to "fall away," the booster is OK; if no effect is felt under foot pressure, replace the booster.
- How is the transmission vacuum diaphragm tested?
- First check and correct the automatic transmission fluid level, then verify at least 10 inches (Hg) of vacuum to the diaphragm. If the condition still exists, attach a vacuum tester and vacate to 18 inches (Hg); if the diaphragm will not hold 18 inches of vacuum (the needle drops), replace the diaphragm.
- What does a spongy brake pedal under foot pressure indicate?
- Per this document, if pedal operation feels spongy under foot pressure, you should bleed the hydraulic system of air.
- How do I check the vacuum reserve tank for a leak?
- Disconnect the vacuum supply line at the engine, plug the line, disconnect the outlet line from the tank, attach a vacuum tester, and vacate the tank to 10 inches (Hg). If the reading drops, repair or replace the tank; if it holds, test the check valve, if the reading drops when the plug is removed from the supply line, replace the check valve.
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