Brake question . . .

[Mick]

My neighbor has a '73 Mustang that the brake pedal started going to the floor, so he changed the master cylinder. After that, if he drove the car regularly, the brakes were fine, but if he parked it for a week or more, when he got in, the pedal would go to the floor. If he pumped the pedal about 20 times, he got good braking that would not decline while he was cruising around. He changed all four wheel cylinders, and the condition is unchanged from when he changed the master cylinder.

I'm guessing that the master cylinder he bought is a chinese piece of shit, and isn't working properly. He agrees that is most likely. But I said I would ask you folks for any other ideas before he changes the master again. So, what say you?

As always, sarcastic comments, personal insults, and general tomfoolery are welcomed and encouraged. Thanks.

[mrvette]

Search out the brake lines, and IF you find an odd kind of fitting between the m/cyl and the final Y to the front/rear brakes, THAT may well be the issue.....

on my '72 vette it was an issue and eliminating it has made good brakes, very consistent ......

[Big bob]

Did he bench bleed it first?

[Aerovette]

Originally Posted by Mick

My neighbor has a '73 Mustang that the brake pedal started going to the floor, so he changed the master cylinder. After that, if he drove the car regularly, the brakes were fine, but if he parked it for a week or more, when he got in, the pedal would go to the floor. If he pumped the pedal about 20 times, he got good braking that would not decline while he was cruising around. He changed all four wheel cylinders, and the condition is unchanged from when he changed the master cylinder.

I'm guessing that the master cylinder he bought is a chinese piece of shit, and isn't working properly. He agrees that is most likely. But I said I would ask you folks for any other ideas before he changes the master again. So, what say you?

As always, sarcastic comments, personal insults, and general tomfoolery are welcomed and encouraged. Thanks.

You made no mention of shenanigans. Are you a shenanigist?

[GTOguy]

Air is in the system or getting in. I would re-bleed the system, starting with the master, and go from there. As stated, could be a metering valve (located below the master) but not as common. Also the MC could be the wrong one and the stroke could be off. My gut tells me he never got all the air out in the first place.

[Mick]

Originally Posted by mrvette

Search out the brake lines, and IF you find an odd kind of fitting between the m/cyl and the final Y to the front/rear brakes, THAT may well be the issue.....

on my '72 vette it was an issue and eliminating it has made good brakes, very consistent ......

I'm not sure I understand this. Should be a dual system in 1973, no Y between the front and rear brakes.

Originally Posted by Big bob

Did he bench bleed it first?

I did not ask him that. Good question. Thanks.

[mrvette]

Originally Posted by Mick

I'm not sure I understand this. Should be a dual system in 1973, no Y between the front and rear brakes.



I did not ask him that. Good question. Thanks.

NO, similar to my '72 vette, front m/cyl goes to rear piston on the m/cyl, and rear to front piston referring to the direction of the CAR, not piston actuation.....call it the forward piston?? each side has equal application, and so the main line going there, needs split into a Y to feed each side the same.....
IF you car has a different system, it's not my knowledge.....I just can't imagine anything much different ....
My car had a front/rear proportioning valve in it, mounted lo down under the master cylinder, when I went to getting rid of the vacuum booster I put in a hydraulic setup running off the power steering pump.....and so that silly proportioning valve to do with F/R application pressures was not needed....

[markids77]

Flexible brake lines deteriorate from the inside and can cause the symptoms described. I recently helped troubleshoot the same issue on a late model F150... owner changed master, pads, rotors, and calipers only to have soft pedal a day after bleeding. I watched his front lines as he pumped the brakes and I could actually see the lines balloon under pressure.

[SQUIRMIN VERMIN 84]

Originally Posted by markids77

Flexible brake lines deteriorate from the inside and can cause the symptoms described. I recently helped troubleshoot the same issue on a late model F150... owner changed master, pads, rotors, and calipers only to have soft pedal a day after bleeding. I watched his front lines as he pumped the brakes and I could actually see the lines balloon under pressure.

I had the same experience with my old 76 3/4 Chevy van.
replaced both hoses and no problem since.

I cut open the hoses and could see the chunk of rubber lining
that plugged the hose under pressure.

[Mick]

Originally Posted by mrvette

NO, similar to my '72 vette, front m/cyl goes to rear piston on the m/cyl, and rear to front piston referring to the direction of the CAR, not piston actuation.....call it the forward piston?? each side has equal application, and so the main line going there, needs split into a Y to feed each side the same.....

Gene: I appreciate your efforts here, and it's probably just my ignorance, but I have no idea what this means. Can anyone translate this into "common dumbass", the regular language of car hobbyists?

[Frankie the Fink]

As mentioned air is the culprit; if it were the rubber lines; the fault would be consistent and not change as the car is driven... I would look elsewhere but its certainly a simple enough matter to change the rubber hoses and rebleed.

[Yadkin]

Originally Posted by Mick

Gene: I appreciate your efforts here, and it's probably just my ignorance, but I have no idea what this means. Can anyone translate this into "common dumbass", the regular language of car hobbyists?

Change out the brake hoses.

He also may have a leak in one of the steel lines. These lines were uncoated from the factory so rust from the outside in. DOT3 fluid absorbs water so if not changed periodically, the lines will rust from the inside. Easy to diagnose- just look for the wet spot.

[mrvette]

OK, my language is confusing.....in the m/cylinders I am familiar with, there are two independent sections/pistons.....they in series with each other, sliding in the same bore through the casing-typically of iron.....so the first piston to get moving from the pedal is the one toward the rear of the car, and so it typically activates the front brakes, the one further forward/deeper into the housing typically activates the rear brakes.....

so in the case of my '72 vette, the deeper/forward section goes to the Y mounted toward the rear of the car, and splits to each side.....

same Y for the front, off the first activated piston, as mounted it's toward the rear of the car in it's location, in the housing.....

[Mick]

Originally Posted by mrvette

OK, my language is confusing.....in the m/cylinders I am familiar with, there are two independent sections/pistons.....they in series with each other, sliding in the same bore through the casing-typically of iron.....so the first piston to get moving from the pedal is the one toward the rear of the car, and so it typically activates the front brakes, the one further forward/deeper into the housing typically activates the rear brakes.....

so in the case of my '72 vette, the deeper/forward section goes to the Y mounted toward the rear of the car, and splits to each side.....

same Y for the front, off the first activated piston, as mounted it's toward the rear of the car in it's location, in the housing.....

OK, I get this. Thanks.

Now I have to go back and re-read the rest of what you wrote, and think about it some more!

[DJ_Critterus]

Originally Posted by Mick

OK, I get this. Thanks.

Now I have to go back and re-read the rest of what you wrote, and think about it some more!

and hire a Jive Translator

[GTOguy]

Bench bleed the master and then bleed the brakes at each wheel---Pass rear, driver rear, pass front, driver front. Brake hoses don't cause systems to 'pump up' and work. They cause crappy pedal all the time of expanding, or hard pedal when they collapse internally.

[Yadkin]

Originally Posted by GTOguy

Bench bleed the master and then bleed the brakes at each wheel---Pass rear, driver rear, pass front, driver front. Brake hoses don't cause systems to 'pump up' and work. They cause crappy pedal all the time of expanding, or hard pedal when they collapse internally.

The hose fittings could be leaking.

[GTOguy]

Originally Posted by Yadkin

The hose fittings could be leaking.

If they were, he'd have a visible leak and it would not pump up as stated.

[Rodnok1]

Originally Posted by GTOguy

If they were, he'd have a visible leak and it would not pump up as stated.

Might not show a leak but doubt they ever get pressure.
I've had bad fittings not leak but suck air and never get them bled properly. It was from some fuktard way over tightening fittings at flex/hard lines. Mangled it. It did not leak a drop.
I bet it's a bad replacement master cylinder.
Nobody has asked but does it have power booster?

[Yadkin]

Originally Posted by mrvette

OK, my language is confusing.....in the m/cylinders I am familiar with, there are two independent sections/pistons.....they in series with each other, sliding in the same bore through the casing-typically of iron.....so the first piston to get moving from the pedal is the one toward the rear of the car, and so it typically activates the front brakes, the one further forward/deeper into the housing typically activates the rear brakes.....

so in the case of my '72 vette, the deeper/forward section goes to the Y mounted toward the rear of the car, and splits to each side.....

same Y for the front, off the first activated piston, as mounted it's toward the rear of the car in it's location, in the housing.....

Fords of that era are different. The master has two ports that go to a combination valve. If the car has front disc/ rear drums then one-half of the master is larger (the front). The CV is brass and mounted close to the master.

The CV has three outlets. Two go to the fronts, left and right. Each go through a steel line to the brake hose at each front wheel. The ins/ outs are all different sizes to make the factory install fool-proof.

The third CV outlet goes to a steel line, to a hose close to the differential housing. On the solid rear axle the hose attaches to a brass tee then steel lines to each wheel.

The CV applies pressure to the rear brakes first, then to all four with most of the braking force applied to the front. CVs are matched to the brake and chassis configuration.