98 D1 Hard brake pedal

[dkfrizzell]

I recently picked up a new 98 D1 with a very stiff brake pedal. You have to push harder than normal to get it to stop.

Pads are good, fluid was dark so I flushed and bled the system (which helped a little), but it is still harder than it should be (this is my second D1).

I checked and there seems to be good vacuum at the plenum. While idling, I disconnected the vacuum line from the booster and the pedal became even harder to push so I think I can rule out the booster.

Which leaves the master cylinder or the ABS pump. Anything else I can diagnose further or do I just need to start swapping parts?

Thoughts?

Thanks,
Dave

 

[redhawkrovers]

Vacum leak where master cyl. mates with booster should have an O- ring on master cyl.
ask me how i know this.

 

[Mike_Rupp]

My guess is a bad proportioning valve. It's relatively easy to overlook the proportioning valve when brake problems pop up. I was at my wits end on my truck and replaced a few things and nothing helped. The valve ended up being bad and it made a huge difference.

 

[dkfrizzell]

I will take a look at both.. where is the portioning valve located?

 

[Mike_Rupp]

Drivers side firewall below the booster. Get the rave manual. It explains how the system is plumbed.

 

[discodriveradam]

Are you losing any brake fluid that you can tell? Is your brake light coming on on your dash?

 

[dkfrizzell]

Fluid level is holding steady. No leaks.

 

[bmn1965]

Worked on a truck a few weeks ago with same problem and it was the master cyl it was not leaking down.but It was letting the fluid bypass in side the cyl.

 

[dkfrizzell]

Checked the vacuum again and it looks good. Also swapped out the portioning valve. No change.

I guess I will try swapping the master cylinder next.

 

[ptschram]

Calipers? Pistons seized up?

 

[dkfrizzell]

Follow up: I swapped out the master cylinder, then the booster, then the ABS pump. None made a difference.

Today I put on new calipers and that solved the problem. Thanks all for the suggestions.

 

[agbuckle98]

did you do the D90 front caliper conversion? I did that to my GF's truck when she needed a full-on brake job, and didn't tell her. First time she stomped on the pedal she just abut put a few teeth marks in the steering wheel!

 

[dkfrizzell]

I did the conversion on my old D1 because I used it to tow my IIA. This one I left stock because it won't be seeing any heavy duty towing.