Stiff Brake pedal

alexk89 · Apr 1, 2008

Ok here is my deal. I own a 89 Foxbody Mustang. I have just replaced my master cylinder because it was going bad. I have installed a new Master Cylinder but the brake pedal is very stiff now. Upon applying the brake about an 1" the brake begins to engage. I have full stoping ability but it is hard to push. It isn't smooth or easy to push. I have done a quick google search and found nothing along with a quick search here on stangnet as well. I was hoping that maybe somebody could help me out on adjusting this problem. Just quick thought but could it be the "Pressure Differential Valve" or maybe the "Pushrod" is to far out?(Just taking a guess after doing some research) any help will be much appreciated.

krallman · Apr 1, 2008

This means that your not used to the new one.

Redfire 05 · Apr 1, 2008

You can try adjusting the brake booster piston rod. If you remove the two master cylinder bolts and pull the master cylinder forward carefully you can access the piston. You should be able to do this without removing the brake lines if you are careful. I don't remember what size wrench you need, but you turn the rod to adjust it. I think you will need to shorten the piston to reduce the stiffness of the pedal.

alexk89 · Apr 2, 2008

Redfire 05 said:
You can try adjusting the brake booster piston rod. If you remove the two master cylinder bolts and pull the master cylinder forward carefully you can access the piston. You should be able to do this without removing the brake lines if you are careful. I don't remember what size wrench you need, but you turn the rod to adjust it. I think you will need to shorten the piston to reduce the stiffness of the pedal.

Ok thank you. I will give this a try.

HISSIN50 · Apr 2, 2008

Is the booster fully functional and receiving a full vac signal?

Redfire 05 · Apr 2, 2008

Something else occurred to me. Did you do anything else to the brake system besides replace the master cylinder? Because adding bigger brakes (for instance Cobra rotors and calipers) can drastically change the feel of the brake pedal. When I put SN95 cobra brakes on my 86' I also had to change to the SN95 brake booster and master cylinder. I started out with the SN95 GT master cylinder, but the pedal feel was really stiff so I went with a SN95 cobra master cylinder and the brake pedal feels great. They have different sized pistons which makes a huge difference in pedal feel.

alexk89 · Apr 3, 2008

HISSIN50 said:
Is the booster fully functional and receiving a full vac signal?

I'm not sure how to check this.

alexk89 · Apr 3, 2008

Redfire 05 said:
Something else occurred to me. Did you do anything else to the brake system besides replace the master cylinder? Because adding bigger brakes (for instance Cobra rotors and calipers) can drastically change the feel of the brake pedal. When I put SN95 cobra brakes on my 86' I also had to change to the SN95 brake booster and master cylinder. I started out with the SN95 GT master cylinder, but the pedal feel was really stiff so I went with a SN95 cobra master cylinder and the brake pedal feels great. They have different sized pistons which makes a huge difference in pedal feel.

No just the master cylinder because it was the only thing that needed to be replaced. I didn't upgrade anything on the car.

HISSIN50 · Apr 3, 2008

alexk89 said:
I'm not sure how to check this.

As far as feel goes, if you have ever shut a car off (or towed one with a tow rope) and used the brakes a few times without the engine running, that's kinda how it feels with a bad booster. The brakes have the potential to work as good as always but only if you have massive legs.

I use a vac gauge/pump (like a Miti Vac) to check things. Disconnect the big fat vacuum line to the booster. Connect the pump to the booster and see if it will hold vacuum. Then with the engine running, put the gauge on the vacuum line itself and see if you have about 20" hg of vac. The booster sees manifold vac and that's what a healthy stockish engine produces. Note that when you get to near single digit manifold vacuum, that's when the brake booster cannot keep up (even if it functions properly).

Good luck.

alexk89 · Apr 4, 2008

HISSIN50 said:
As far as feel goes, if you have ever shut a car off (or towed one with a tow rope) and used the brakes a few times without the engine running, that's kinda how it feels with a bad booster. The brakes have the potential to work as good as always but only if you have massive legs.

I use a vac gauge/pump (like a Miti Vac) to check things. Disconnect the big fat vacuum line to the booster. Connect the pump to the booster and see if it will hold vacuum. Then with the engine running, put the gauge on the vacuum line itself and see if you have about 20" hg of vac. The booster sees manifold vac and that's what a healthy stockish engine produces. Note that when you get to near single digit manifold vacuum, that's when the brake booster cannot keep up (even if it functions properly).

Good luck.

Ok Thank you.

Hyland33 · Apr 4, 2008

My brake pedal was really stiff because my rear brakes were frozen from lack of use...once i fixed them the pedal seemed a bit softer

alexk89 · Apr 7, 2008

Hyland33 said:
My brake pedal was really stiff because my rear brakes were frozen from lack of use...once i fixed them the pedal seemed a bit softer

Well i do think that my rear left brake is dragging alil. It seems to get hotter on that side like it is being used more or is on alil tighter than the right rear side. Is there a way of adjusting this?

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