Brake pressure specs

[aslanefe 333]

Does anyone know what the brake pressure specs are for front disk, front drum, rear drum, with booster and without booster for 69 and 70?

[barnett468 418]

I don't quite understand exactly what "specs" you are looking for.

What are you wanting to achieve?

 

 

[aslanefe 333]

I know the pressure depends on how strong the person pressing the pedal is. I get about 700 psi to front (with early style 4 piston calipers), 600 to rear drums with a 1 inch MC with no vacuum to the booster. And about 1400 psi to front and 1000 psi to rear with 18-20 inch vacuum. The MC, calipers and booster are not original 69 equipment, pedal is power brake pedal hang high. I was trying to see if the pressures I have are in acceptable range as the rears would lock but not the fronts. Fronts have new pads, rotors; rears have new shoes and original drums that look very clean and true. Shoes are Wagner Z151R, no idea on the pads as they came with the kit and I did not see any markings on them.

[barnett468 418]

On 3/11/2019 at 11:06 AM, aslanefe said:

I know the pressure depends on how strong the person pressing the pedal is. I get about 700 psi to front (with early style 4 piston calipers), 600 to rear drums with a 1 inch MC with no vacuum to the booster. And about 1400 psi to front and 1000 psi to rear with 18-20 inch vacuum. The MC, calipers and booster are not original 69 equipment, pedal is power brake pedal hang high. I was trying to see if the pressures I have are in acceptable range as the rears would lock but not the fronts. Fronts have new pads, rotors; rears have new shoes and original drums that look very clean and true. Shoes are Wagner Z151R, no idea on the pads as they came with the kit and I did not see any markings on them.

do you have a factory proportioning valve and is it working properly?

do the rears lock up first both with and without the power booster?

700 psi to the front is too little for discs unless you can bench press 400 lbs with your legs.

 

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[aslanefe 333]

I have a GM style prop valve/distribution block.

Did not drive it without booster, but with booster nose dives down first then rears lock up; fronts do not. When going in reverse, the fronts lock up, rears don't.

I can't bench press 400 lbs anymore, that's why I went with power booster. 

I would think that prop valve is working as I get more pressure to the fronts (1400 psi fronts, 1000 psi rears with booster). If it matters, the brake warning light on dash comes on while bleeding the brakes and goes out after bleeding and applying the brake once or twice.

 

[barnett468 418]

39 minutes ago, aslanefe said:

I have a GM style prop valve/distribution block.

Did not drive it without booster, but with booster nose dives down first then rears lock up; fronts do not. When going in reverse, the fronts lock up, rears don't.

I can't bench press 400 lbs anymore, that's why I went with power booster. 

I would think that prop valve is working as I get more pressure to the fronts (1400 psi fronts, 1000 psi rears with booster). If it matters, the brake warning light on dash comes on while bleeding the brakes and goes out after bleeding and applying the brake once or twice.

 

ok you can change the percentage of fluid that goes to the rears by installing an adjustable aftermarket prop valve in the rear line then reducing the pressure to the rears or installing smaller rear wheel cylinders if they are available.

if your light is on, it should not go off by itself if you push on the pedal but as long as it is off, i would guess it is ok.

are you certain that there is no air left in the lines?

does it stop abnormally poorly?

do the rears lock up under light to moderate (normal) pedal pressure?

 

 

 

[aslanefe 333]

Pedal is firm and does not go in all the way (except while bleeding); so I do not think there is air in the system. It stops good. Rears don't lock up under normal pedal pressure. It locks up when I brake real hard. Adjustable valve in the rear line will reduce the pressure to rear so they lock up later but in my case looks like I need more pressure to fronts as my problem is fronts not locking up at all. The reason why I was asking for pressure values for rear and front was to see if my pressures are comparable or not. If they are, then I would suspect that the pads I have are the source for not being able to lock the fronts.

[barnett468 418]

11 hours ago, aslanefe said:

Pedal is firm and does not go in all the way (except while bleeding); so I do not think there is air in the system. It stops good. Rears don't lock up under normal pedal pressure. It locks up when I brake real hard. Adjustable valve in the rear line will reduce the pressure to rear so they lock up later but in my case looks like I need more pressure to fronts as my problem is fronts not locking up at all. The reason why I was asking for pressure values for rear and front was to see if my pressures are comparable or not. If they are, then I would suspect that the pads I have are the source for not being able to lock the fronts.

unfortunately it's not quite that simple, but if you have crappy shoes in the front, or ones that are for road racing etc, then yes, pads with higher initial grip when cold will help.

if it takes an unusual amount of force to lock the brakes in general with the power booster, then you need a smaller master or a higher power booster as well.

also, i'm not positive how the gm prop valve system works works and this may be contributing to the problem. i would have used a 69 or 70 ford prop valve setup.