Recently rebuilt my power steering cylinder and took my 1970 for a test drive. At then end, i noticed my brake pedal was extremely soft. When I got home I opened the master cylinder and the front reservoir was completely empty and the rear one over the fill mark. The gasket was also out of shape. Figured id ask for insight before I start the process of tracking problems.
rear brakes are possibly leaking. Remove drums & inspect. New gasket or new mas cyl too. If no fluid found then the master cyl is leaking into the booster…
You don’t say whether the brake system is power or manual and you don’t say if it is disc or drum. More information is needed to help you.
I do have a power system. Ive got disc brakes in the front and drum in the back.
In that case remove the master cylinder. Likely it is leaking into the brake booster. I would have both rebuilt.
Do you suspect brake fluid has migrated from the front chamber to the rear? That would be unusual because the pressure in the rearmost (primary) cylinder is typically a few bar higher than the pressure in the front (secondary) chamber.
Definitely start by looking for leaks at the rear wheel cylinders. If there is any fluid on the brake shoes you need to replace them and clean any fluid from the drum. Contaminated shoes grab and are dangerous, especially on the rears.
Did you fill the rear chamber before replacing front pads? The fluid in the chamber that works the disc brakes will drop as the pads wear. A common mistake is to top off the fluid, replace the pads at a later time, which when you push the pistons back into the calipers causes the fluid to be higher than full.
If the master cylinder is leaking into the booster you should be able to easily see that by separating the MC from the booster and looking for fluid at the rear piston/push rod interface.
When the brake fluid kept going away in my '67, non powered system, I found the seal in the MC was bad. It was leaking out through the plunger hole. I had brake fluid all over both sides of the fire wall. Got a new MC from RockAuto, no problems since.
Rick
BTW, I had replaced the original MC a couple years back when I upgraded to front discs (from a Lincoln Versalles), (and yes, I added the proportioning valve at the rear axel), but RockAuto replaced the MC under warranty.
Thanks for all the help. Looks like the master cylinder seals are just going out and sucking fluid into the booster. I couldn’t find any other leaks. So plan is to replace the cylinder and see if that does it.
…and the booster? The fluid that leaked into the booster needs to be neutralized. How? Good question. The reason is that it may work today but in time the brake fluid will damage the diaphragms inside and then your doing it all over again.
Well i guess ill replace the booster while im at it!
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There are no good replacements available. Only crappy ones that don’t work. Send your original to Booster Steve and it will get rebuilt properly.
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Scott Taylor and I have been using this one and we have had good luck so far.
70 uses the Bendix unit without teardrop pushrod. WCCC has the Booster Steve rebuilds if you don’t have the correct core:
Brake Booster - Power - Bendix - PREMIUM - Rebuilt - PRE-PAY CORE CHARGE ~ 1970 Mercury Cougar / 1970 Ford Mustang ( 1970 Mercury Cougar, 1970 Ford Mustang ) at West Coast Classic Cougar :: The Definitive 1967 - 1973 Mercury Cougar Parts Source
I need to try one of those!
So I did go ahead and order a new MC and booster. Figured id go ahead and retrace all my lines to double check everything. While doing so, I noticed my pressure differential valve assembly seems to be leaking. I assume this is also contributing.
That could be the entire problem. Too bad you chose to shotgun parts.
Your 70 should have a combination proportioning valve and differential pressure switch, AKA “combo valve”. Here is the rebuild kit. It’s not a tough job.